Transactionize reservations checkout (?)

[dgoerger]

In case one fails checkout (although with @ebmaher 's validations, none should), and there is for example a Camera + Tripod, we don't want the tripod to be checked out but not the camera (b/c that would be silly and absurd..).

Of course maybe we only want rollback action for associated eq models? Use-case: I try to check out a Camera + Tripod + Laptop, only the laptop and tripod pass, but realistically the tripod is useless without the camera although I can still make use of the laptop.

Some eq models which are associated, though, aren't as dependent on each other as is the Camera + Tripod case; so we would still want an option to check it out regardless of the fact that its associated model cannot be checked out (a can_checkout? override).

Guys if this would result in too much / complicated code debt, we should remove the comment from the reservations controller:

  # transaction this process ^downarrow

[dgoerger]

Alternatively we could call .valid? somewhere in the checkout action and implement proper error processing on the receipt page for invalid reservations (which should never exist).

[mnquintana]

Where do we stand with this one now? Is this related at all to the Great Cart Refactor of 2014?

(EDIT by @dgoerger: fixing the link)

[dgoerger]

child of #666.

[squidgetx]

let's just merge this into #666, since @shippy has already transactionized the process in his branch.