Add Drupal logo to the toolbar

[ckrina]

Description

We're adding more menus into the toolbar, so we will be adding headings above each menu.

Activities in this ticket

Add a heading to the existing administration menu (see screenshot example).

• The text doesn't need to be in caps for now. The important part in this ticket is the markup, not the styles.
• Use a markup similar to the one coming from block--system-menu-block.html.twig so it's easier to integrate later with the block and menu systems in Drupal. Example code:

<nav id="this-menu-id" class="" aria-labelledby="title-id" role="navigation">
 <h2 class="" id="title-id">Administration menu</h2>
 <ul class="">
       <li class=""><a >Link text</a></li>
 </ul>
</nav>

[sveta1x]

conflict is fixed, styles are updated to postcss. Ready for QA #5

[sveta1x]

approved, merged