Add floor of zero to availability counts in calendar in catalog

[orenyk]

BMEC is showing negative availability on iPads. There are 20 equipment items, 6 of which are deactivated and the rest of which are checked out with future due dates.

[orenyk]

I thought we might have some overlapping reservations that weren't checked out; trying to find them now (woot Active Admin)!

[orenyk]

Yup, there are three reservations that overlap with today's date that haven't been checked out, hence -3. I'm not sure how those were created (either we had some approved requests that failed availability validations or just overriden validations), but we should give the availability a floor of zero just for clarity. In the future we can work on the wait list functionality (see #517).

EDIT It's also possible that equipment was deactivated for reimaging, reducing the number of available equipment items (so they would have been valid when created).

[orenyk]

This will be the final change for v3.4.5; I'll try to do this in GitHub since I'm not in my dev environment but if we can get it reviewed and merged today I'll push to DEV tonight.

[orenyk]

We should also have a warning if a deactivation will prevent existing reservations from being fulfilled; I'll create a separate issue for that and it will go in v4.x.