[fix bug 1033306] Allow Mozillians to update their legacy vouch by re-vouching.

...uching.
What's going on in this patch? Removing this unique constraint is a really bad idea. If we want to update legacy vouches, is_vouchable should return True if you have a vouch with no description on the account in addition to the cases it has now, and vouch() should check to see if you've vouched the user before, and if you have and the vouch has no description, it should update that vouch with the new description. It should not add a new vouch. You should NEVER have multiple vouches for a user.