Small left menu improvements

I got stuck on the notification dot in https://gitlab.com/gitlab-org/gitter/webapp/-/merge_requests/1850

So for now I'm sharing some initial refactoring I've done so I can plug in the new Announcements logic easier.

The motivation for these changes was use case of adding a new button to the left menu. This prepares the ground for the next commit https://gitlab.com/gitlab-org/gitter/webapp/-/commit/bee6f0092c9a094af622df88c866fc0e6f01b8d1?merge_request_iid=1850 but IMO these changes have merit on their own.