Restrict who can associate repos with groups

[dontcallmedom]

For good or bad reasons, the current instance of the repo manager lists repos that were more or less randomly added and associated with W3C groups (e.g. for testing purposes).

To keep our data clean, we should:

• make it easier to remove repos (Allow to un-manage a repository #6)
• restrict who can add repo; possibly:
  • staff can add repo for any group
  • WG/CG chairs (participants?) can add repo for their own groups

(per discussion with @plehegar )