Invites part II

[sudara]

Followup of #1088

• Users see a new "invite" menu item
• Users (just like mods) can email a single use invite link
• Users can also generate a single use invite link
• An email goes out with the invite link attributing the action to the alonetone user who sent it
• Users can see a list of accepted invites, linking to the created user
• Users can see a list of pending invites

Questions:

• Do we want to prevent email addresses from being invited more than once?
• Do the invitations expire for some reason?
• Is there a maximum number of open invites?
• Do users need to be able to delete invites?