Accepting "join language team" request should warn if already a member

1. Users requests to join a language team
2. An admin adds user to language team directly (not via requests page)
3. Another admin goes to the requests page and accepts the join request

Expected result:
No requests visible, or "Note: this user is already in the team"

Actual result:
Request appears to be accepted. Clicking "accept" presumably doesn't change any language permissions.

If someone fulfils a request manually, Zanata should dismiss the request, whether the permission change was done through the requests system or not. Or failing that, the requests system should show a warning when viewing the request.