We do the wrapping in an array in represent_hierarchy for children.

When finding children for a group

And document what extra queries are still being performed.

The children are lazy-loaded when expanding

The possible cases are:
 - [Array, Array]
 - [Array, Hash]
 - [Array, GroupHierarchy]
 - [Hash,Hash]
 - [Hash, GroupHierarchy]
 - [GroupHierarchy, GroupHierarchy]

When fetching children for a group with a filter, we will search all
nested groups for results and render them in an expanded tree

Cleanup Select2 dropdown styles

Closes #37279

See merge request gitlab-org/gitlab-ce!14557

Makes `@mentions links` have a different styling for better separation

See merge request gitlab-org/gitlab-ce!14377