Add custom lecture times to classes

[Balint-R]

Some classes (e.g. 21G foreign language classes) allow you to swap between sections and go to a different lecture section based on the day of the week. Hydrant does not currently support mixing different sections.

The simplest way to fix this is probably to add a way to input custom times for a subject (similar to what you do for custom activities). This would also address cases where instructors don't update section information on subject listings (e.g. I believe 6.1903 last semester had a 5-7.30 lab section that was never posted to subject listings).

You should also be able to assign different locations to the different time slots.