Added 100% width to open dropdown menus on mobile

What does this MR do?

Sets dropdown-menus to be 100% width on small screens.

Are there points in the code the reviewer needs to double check?

Do we want this on all dropdown menus or only dropdown menus activated by full-width buttons? /cc @dzaporozhets

Why was this MR needed?

Mobile UX.

What are the relevant issue numbers?

Closes #18858 (closed).

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

app/assets/stylesheets/framework/dropdowns.scss

@@ -22,6 +22,9 @@
 .open {
   .dropdown-menu {
     display: block;
+    @media (max-width: $screen-xs-max) {
+      width: 100%;
+    }
   }
   .dropdown-menu-toggle {