Test that everything works as described.
Do we lose anything by removing the alert status?
Production buildings don't have the alert status, meaning newly created units won't auto-garrison. This was a closed ticket, #2342, so the change counts as a regression, but I don't think it's a big deal. In the second patch I play on removing levels, which counts as another regression, but I don't think they're necessary. (We could do both levels and classes, although the GUI might get complicated.)
The alert status on units was used when finding new garrison targets when the current target was full, but now we use causative's idea of keeping track of the reserved spots so that's unnecessary. I've implemented this very simply, with units closest to the alert raiser having the first choice of garrison holders. Because units might garrison in a building outside the alert range, when we unring the bell we have to use a larger search radius for garrison holders.
There might be performance issues with redoing the calculations repeatedly if people keep hitting the alert button (I haven't changed the gui yet), so if that seems like a problem we might need to keep track of the time and not redo the calculation if it's on the same turn or whatever.