Check for over-reserving equipment when extending reservations

[orenyk]

If an admin extends a reservation manually (e.g. via #edit instead of #renew), they may cause there to be an equipment shortage for upcoming reservations. This should be checked during the #edit method and a JS prompt should be used to confirm (much like #983).

[orenyk]

Ok, this is going to be trickier than I thought. We'll only be able to check availability once an end date has been selected. We might want to do some JS magic much like we did with switching equipment items, but I'm not sure if we should generate an indefinite availability array...

[orenyk]

I think I'm going to want to just find the next day that will cause negative availability and check that the new end date is prior to that date.

[caseywatts]

Moving to wish list to save time.

[orenyk]

Note that this was kinda started in the 1058_... branch.