user invites on-prem: design improvements for admin communication

Created by: slimsag

We will need to update the frontend designs for on-prem user invites to make it clear to admins:

• That we've added user-invites on-prem, and how they work.
• How to disable them if they wish to.
• That inviting someone merely sends them a link as if you'd done it yourself, there is no special access granted.
• That we find collaborators via Git repository history, so if you see someone unexpected as a suggestion that's where it came from. Not a security concern.

May slip into G&I iteration 4.