add DC middle school program to application form

Adding this to the application had three pieces:

1) the database field, represented by the migration. This also adds it to the csv exports automatically.
2) The UI checkbox in the view. The saving is handled automatically too btw by the presence in the database and the existing generic save javascript.
3) Making it possible to enroll them. This is done in the admin controller edits.

#3 warrants a wee bit more explanation: DC was added following the same pattern as NYC, but now that there's three, a simple if/else wasn't enough. Instead, I replaced that with a loop of possible columns to check - also making this more generic in the process. We only need to edit that array to add more programs, assuming a person can still only be accepted into a single program at a time.

Existing spreadsheets should still just work since I added it to the right where before there was nothing. Then, when we're ready to take this live, all abby has to do is add the two columns to the sheet and the upload should just work.