Improve maintenance page

Here is a first patch, added maintenance page template that is using default structure from the main page template. Added a bit of CSS in a new maintenance library and inline SVG work in progress image so things don't look that boring ;)

Added small vertical margin for a header logo so it does not stick to viewport top border.

Here are some screenshots

Before	After

Added more blocks in template so override is easier in child themes.

It seems it's better to use padding-top then margin-top for div.region-brand element.

There is a possible problem when the sticky navbar is active - it seems it is running it's logic and applying a negative margin. Check this and fix it.

Removed navmenu class from template and block class from navbar-brand selector which was enough to fix previously mentioned margin-top/padding-top rule - we do not need it any more.

Active sticky navbar was false positive, so not a bug.

Added new bs_base.maintenance_image.path theme configuration which we can use to store maintenance image path. Now custom projects can easily change this image.

Refactored maintenance-page template to extend from bs_bootstrap page template.

Image can be removed now, optimized CSS class names a bit.

Forgot to move maintenance image to bs_base/images.

Moved bs_versions above so it does not conflict with Drupal packaging info lines - now we can use this in composer against the latest release.

Documentation explaining this new feature is added in https://www.drupal.org/docs/8/themes/bs-base/additional-features/mainten....

Removed extra container CSS class for div.region-navbar element. Committed.