Add ability to pin groups (and repos?)

This is probably an addition to #20218 (closed) and #26968 (closed) and may be considered together with them.

In self-hosted GitLab instances used by teams, most of the groups may be project-based while a limited number of others can be pretty general ones. Those general ones can contain repos with commonly used documents, templates, project boilerplates, etc. Even when there is an option to sort the groups by a name, it'd be useful to have some of them attached to the top of the list in some custom order. This is pretty much like on old-day forums or in a Mac's Dock in the bottom of the screen, where you can pin and arrange the icons of the most-used apps.

Would it be possible to allow GitLab admins to pin groups and manually order them among the pinned ones? In this case these groups will be always easy to navigate to by everyone and it will also not be necessary to add every new team member into each of the general-purpose internal groups in order for them to become visible on everyone's dashboard for groups.

The same principle can be also applied to projects inside groups I guess. Group owners can be allowed to pin a few projects in a very big group to ease everyone's navigation.

What do you guys think?