Generate broadcasts from active shows with a "daytime" rule

[gsf]

To start with, this will be a script to be run manually that generates broadcasts for three months out.

TODOs for separate issues:

1. Keep showStart and showEnd as non-editable fields in each broadcast. When broadcasts are generated, check these fields so we don't create duplicates. Also, when daytime is changed, if these fields are the same as the broadcast's start and end (and the broadcast is in the future), then modify all to match the new daytime.
2. Run automatically on bulk load.
3. Run automatically on show save.

[gsf]

And note that with #12 "daytime" is now "airtime".