refactor schedule REST API

This MR makes a few changes to the schedule REST API:

• It removes deprecated schedule attribute names (dstart, until, tstart, tend, and byweekday) and replaces them with the attribute names from the Schedule model (first_date, last_date, time_start, time_end, and by_weekday).
• It removes the special handling of schedule requests without a solutions object. Instead, clients can omit the solutions object and if no conflicts are detected the schedule is simply created. This avoids race conditions in the client and simplifies our view code a bit.
• Schedule conflict responses are now returned with an HTTP 409 status code.

Fixes #101 (closed).

I will open another MR in the dashboard project that implements the necessary changes there.