Visually improve views exposed forms and entity browser tabs

Here is a first patch. Also added 'views__pager' helper CSS class to mini pager so you can add CSS layout rules easer to it.

Updated patch against latest dev.

1. +++ b/themes/bs_bootstrap/js/entity-browser.tabs.js
   @@ -0,0 +1,46 @@
   + * Navigation bar improvements and tweaks.
   + *
   + * The biggest addition is support for 3 levels of depth for small screens.
   

   Comments need fixing:

   "Use Bootstrap tabs for EB tabs".

2. +++ b/themes/bs_bootstrap/js/entity-browser.tabs.js
   @@ -0,0 +1,46 @@
   +})(jQuery, Drupal);
   

   Missing new line.

3. +++ b/themes/bs_bootstrap/templates/views/views-exposed-form.html.twig
   @@ -0,0 +1,23 @@
   +</div>
   

   Missing new line.

Here is a new patch based on the latest feedback. I think this is good to go now.

Noticed various problems with datetime field. That is improved. Also increased horizontal gap in inline forms and refactored/improved CSS. Removed new div wrapper and form-inline-group CSS class is now added directly to the form element.

Added form-item class to datetime-wrapper and apply margin bottom reset for datetime for all screens.

Added $form-inline-group-gutter SASS variable. Actions should use the same gutter value.

Committed.