[Rails 4] Set up CookieStore

[squidgetx]

We want to migrate away from activerecordsession store to cookie session store. This requires the system administrator (eg, Mike) to maintain a separate secret key for cookie authentication.

[dgoerger]

re fears of potentially overloading the cookie store, the cookie size limit is 4096 bytes (reference).

We should check the current cookie size, diff that with 4096, calculate roughly how many bytes are necessary for the session (session-login + a flash message + reserver_id + cartdates + model_ids and their associated quantities, I think might be sufficient, though we need a bytes number), and then calculate how many model_id / quantity pairs we can add to the cookie before hitting 4096B.

If that number is less than 20, we shouldn't make the switch. @orenyk it might be better to have an even higher upper bound, since we're open source and someone might have a use for more than twenty different models being checked out at the same time (although in a BMEC-type instance I can't imagine anyone ever checking out 20 different models at the same time). We want the number to definitely be higher than anything we can conceive of as reasonable, and then some, to avoid a cookie overload exception, although we can build in hard limits to hopefully avoid outright exceptions (e.g. flash "uhh sorry, technical limitations => max of twenty different models in the cart at any given time. please reserve your equipment now and continue shopping with a second shopping cart after your current reservations are finalized").