Disallow transitive implications. If a -> b -> c already exists, don't
allow a -> c.
Caveat: if b -> c already exists, and we make a BUR for a -> b and a -> c,
the BUR validates even though a -> c is redundant. It only fails
when the BUR is approved.
Previously only the BUR's approver was set when a BUR was approved. Set
the approver for each alias/implication in the BUR as well.
Additionally:
* Refactor `approve!` to take a user instead of just a user id.
* Be mass-assignment permissions aware when setting approver_id.
