Check rights of Invite creator

[joepio]

We recently introduced Invites #134

However, the current implementation makes it possible for users to grant themselves rights to edit things that aren't theirs. To prevent this, we need to perform these checks:

• Check the rights of the Inviter on creating the invite (commit)
• Check the rights of the Inviter on redirecting the Invitee
• Make a test framework for improved flexibility in authorization tests #233
• Set expiration date for invites

I think we need to check this twice, because after an invite has been created, the rights of the creator could have changed.