The list of incidents in the planned maintenance admin details page needs be limited *somehow* for production-amount data

[hmpf]

I was having difficulties figuring out whether I was using the correct maintenance-column while I was testing this, so I hit up the admin for maintenance tasks after having made a very broad task (filter on very popular source type).

That... was a mistake. Eventually, the page finished loading, though I worried about running out of memory for a mo'. It looked like every incident ever was in the incident list for the task, and none was selected.

Originally posted by @hmpf in #1761 (comment)

[hmpf]

The sql query might be snappy but the rendering of thousands of options in the multi-select is definitely not. Easiest fix is probably to exclude any incident with start time before the start of the maintenance, or timedelta x before the start of the maintenance.