Add basic maintenance page

1. +++ b/templates/system/maintenance-page.html.twig
   @@ -0,0 +1,90 @@
   +      <div class="panel panel-default">
   

   I'd rather this not be a panel. A truly "basic" implementation would be just like the normal page: navbar with logo + site name/slogan, a page header and content.

2. +++ b/templates/system/maintenance-page.html.twig
   @@ -0,0 +1,90 @@
   +      <div class="footer-credits">
   +        <p>&copy; {{ "now"|date("Y") }} {{ site_name }}.  {{ 'All rights reserved.'|t }}</p>
   +      </div>
   

   This makes the assumption that the site/content is copyrighted. I'd rather this project not make these assumptions. This can easily be added in a sub-theme as needed.

1. +++ b/templates/system/maintenance-page.html.twig
   @@ -0,0 +1,127 @@
   +{% set container = theme.settings.fluid_container ? 'container-fluid' : 'container' %}
   ...
   +        theme.settings.navbar_inverse ? 'navbar-inverse' : 'navbar-default',
   +        theme.settings.navbar_position ? 'navbar-' ~ theme.settings.navbar_position|clean_class : container,
   

   The database may not be reachable during maintenance. Theme settings will need defaults in case these aren't provided.

2. +++ b/templates/system/maintenance-page.html.twig
   @@ -0,0 +1,127 @@
   +        {{ page.navigation }}
   ...
   +        {% if logo %}
   +          <a href="{{ front_page }}" title="{{ 'Home'|t }}" rel="home">
   +            <img src="{{ logo }}" alt="{{ 'Home'|t }}"/>
   +          </a>
   +        {% endif %}
   +
   +        {% if site_name or site_slogan %}
   +          <div class="name-and-slogan">
   +            {% if site_name %}
   +             <h1 class="site-name">
   +               <a href="{{ front_page }}" title="{{ 'Home'|t }}" rel="home">{{ site_name }}</a>
   +             </h1>
   +            {% endif %}
   +
   +            {% if site_slogan %}
   +              <div class="site-slogan">{{ site_slogan }}</div>
   +            {% endif %}
   +          </div>{# /.name-and-slogan #}
   

   Replace the navigation region with the branding. This is normally a block, but this isn't available in maintenance mode.

   It should be in the navbar header to mimc the normal site.

3. +++ b/templates/system/maintenance-page.html.twig
   @@ -0,0 +1,127 @@
   +        {# .btn-navbar is used as the toggle for collapsed navbar content #}
   +        {% if page.navigation_collapsible %}
   +          <button type="button" class="navbar-toggle" data-toggle="collapse" data-target="#navbar-collapse">
   +            <span class="sr-only">{{ 'Toggle navigation'|t }}</span>
   +            <span class="icon-bar"></span>
   +            <span class="icon-bar"></span>
   +            <span class="icon-bar"></span>
   +          </button>
   +        {% endif %}
   ...
   +      {# Navigation (collapsible) #}
   +      {% if page.navigation_collapsible %}
   +        <div id="navbar-collapse" class="navbar-collapse collapse">
   +          {{ page.navigation_collapsible }}
   +        </div>
   +      {% endif %}
   

   There's no need for this since there are no menu items.

4. +++ b/templates/system/maintenance-page.html.twig
   @@ -0,0 +1,127 @@
   +      set content_classes = ['primary-column',
   +        'col-sm-8',
   +        'col-sm-offset-2',
   +        'text-center',
   +        'vertical-margin'
   +      ]
   

   These are unnecessary. Just set to col-sm-12

5. +++ b/templates/system/maintenance-page.html.twig
   @@ -0,0 +1,127 @@
   +      {# Highlighted #}
   +      {% if page.highlighted %}
   +        {% block highlighted %}
   +          <div class="highlighted">{{ page.highlighted }}</div>
   +        {% endblock %}
   +      {% endif %}
   ...
   +{% if page.footer %}
   +  {% block footer %}
   +    <footer class="footer {{ container }}" role="contentinfo">
   +      {{ page.footer }}
   +    </footer>
   +  {% endblock %}
   +{% endif %}
   

   There's no need for superfluous regions.

+++ b/templates/system/maintenance-page.html.twig
@@ -0,0 +1,73 @@
+          'text-center',
+          'vertical-margin'

Don't center the text please.

1. +++ b/templates/system/maintenance-page.html.twig
   @@ -0,0 +1,63 @@
   +      {% set header_classes = ['primary-column', 'col-sm-12'] %}
   

   Please remove the primary-column class as it is not needed.

2. +++ b/templates/system/maintenance-page.html.twig
   @@ -0,0 +1,63 @@
   +            <img src="{{ logo }}" alt="{{ 'Home'|t }}"/>
   

   The navbar classes should be added to the header/branding containers to make it look decent.

Navbar and branding both need to mimic what already exist:
https://drupal-bootstrap.org/api/bootstrap/templates%21system%21page.htm...
https://drupal-bootstrap.org/api/bootstrap/templates%21block%21block--sy...

It should look like what it does out of the box:

i copied the values from the https://drupal-bootstrap.org/api/bootstrap/templates%21block%21block--system-branding-block.html.twig/8#source.20,25 and i edit the patch .

7.x-3.x still needs an updated backported patch like #24.

Here is an updated patch for 7.x-3.x

I removed the global $theme because i think it is not needed and i replaced the $content_column_classes with .col-sm-12