Add approval update action with undo.
Add boolean method to check if the Thing is owned by a friend of the specified user
Add conditions to BorrowRequest validity

Add checks for Thing availability in user's friend network, and whether
or not a pending request for this user and thing already exists.
Add success return values to request and approval creation
Update borrow_requests create action too use class initializer

The action spawns Approval records to each of the owners of the
requested Thing.
Add "Ask to Borrow" button and pending request info placeholder to Thing views.