Refactor ReservationsController#checkin and #checkout

[shippy]

Superseding #348, #349 and #350, since the refactorings will have to be done in tandem.

Refactorings to do:
For #checkout:

• TOS acceptance validation
• Fix faulty comparisons (e.g. if reservation_hash[:equipment_object_id] != ('' or nil) doesn't do what we'd expect it to do -- reservation_hash[:equipment_object_id].present? is the way to go)
• "Going down the checklist" check (see refactor out similar code in ln 138-140 and ln 225-227 of ReservationController #348)
• Overdue reservations validation (pending Fundamental Rewrite of Validation System #644)
• Move reservation-saving logic into the Cart (pending Fundamental Rewrite of Validation System #644)
• Fix the rescue block to only rescue ActiveRecord exceptions
• Make checkout a transactioned process (Transactionize reservations checkout (?) #225)
• Write full-featured test for the method's full expected behavior (Create full test coverage for ReservationsController #checkin, #checkout and #renew #679)

For #checking:

• "Going down the checklist" check (see refactor out similar code in ln 138-140 and ln 225-227 of ReservationController #348)
• Move reservation-saving logic into the Cart (pending Fundamental Rewrite of Validation System #644)
• Fix the rescue block to only rescue ActiveRecord exceptions
• Make checkin a transactioned process (Transactionize reservations checkout (?) #225)
• Write full-featured test for the method's full expected behavior (Create full test coverage for ReservationsController #checkin, #checkout and #renew #679)

[mnquintana]

\m/