[fix bug 1033422] Add signal to handle UserProfile.is_vouched property.
[fix bug 1033486] Add can_vouch UserProfile attribute
[fix bug 1033420] Allow Admins to add, edit, delete vouches in /admin.
Merge pull request #1015 from glogiotatidis/vouchmodel
[fix bug 1033306] Allow Mozillians to update their legacy vouch by re-vouching.