Limit number of future reservations for a given EquipmentModel

[orenyk]

Again, from BMEC:

Working through extended reservation requests, it occurs to me that it would be useful to be able to limit the number of future reservations a patron can have for a single piece of equipment ... I'd also like to make it impossible, for example, for a single patron to reserve one piece of equipment every single weekend (the way Reservations works, a single patron can for example reserve one of our lenses, of which there is only one each, every weekend for the foreseeable future).

This would be a separate EquipmentModel parameter (e.g. max_future_res) - we can also think about adding a Category limitation as well while we're at it. We'd have to add the parameter to the model (incl. strong params in the controller), set up the relevant methods, and finally set up the validation for the Cart. This is less urgent than #1311, but we can get it done sooner rather than later.