Add 'follow a group' button

Proposal

A nice extension to Make GitLab More Social! Add 'follow a user' bu... (#17772 - closed) would be to allow users to follow a group.

• Add follow button to any group you can visit, can be clicked to follow and clicked again to unfollow
• On your profile page the Following tab (https://gitlab.com/users/<username>/following) can be used to populate the groups the user is following. The current user should also have access to the populated groups otherwise hide the private groups.
• Add a tab to the activity dashboard similar to the followed user tab to show all the group activity.