disable autofill of band performance length from schedule

[binary1230]

• calculations were resulting in incorrect times due to not subtracting setup time
• this calculation also would have broken for bands starting at non-schedule interval times (at :00 or :30 on the hour)
• this is still not a bad idea, but, the schedule itself needs to support bands starting at arbitrary times, and currently doesn't
• also, for main magfest, we usually setup one block for all bands called i.e. "Friday Concerts" so the band plugin would think the performance time was 6 hours for each band

fixes #39

[binary1230]

still testing locally

[binary1230]

tests good locally. going to merge since this is a time-critical thing

[kitsuta]

Change looks good. Hopefully this doesn't, like, undo anything someone specifically asked for last year.