Closes #152

Closes #152

To be honest I'm not sure this is the right way to do it.

The seralizers for Host, Note and Show require the request in the
context, but only the `APINoteViewSet` calls `get_serializer_context`.
The other two `APIHostViewSet` and `ShowSerializer` ingnore it, and the
request needs to be added manually.

- The user or username is no longer "injected" in the view sets
- The user and username are obtained in the serializers from requests

Wrong mix of timezone aware and timezone naive datetime objects

Close #116

Fixes #141

- add `includeVirtual` as GET parameter for `json_playout()`
- refactor `json_playout` to be cleaner

Closes: #120

- remove remaining usage of naive datetime objects
- refactor json_day_schedule and json_playout views
- remove unused methods in TimeslotManager

Only return the next repetition timeslot if there is one

Only return the next repetition timeslot if there is one

Fixes #131

The conflict responses should be send with an HTTP 409 status code as
they’re defined in our documentation. This was not the case as the
dashboard couldn’t handle these error codes, but is now fixed in
dashboard@4a07ad9e.

Until now the steering API client was expected to submit two requests.

1. The initial schedule request, with a `schedule` object
2. A second schedule request, with an additional `solutions` object

This `solutions` object was empty, if no conflicts arose from the
submitted schedule in the first place. But instead of just creating the
requested schedule if no conflicts were detected we still required that
second round-trip, introducing possible race conditions in the client,
if another successful scheduling request was made between requests.

From now on the steering API will create schedules if no conflicts have
been detected and will only require the client to submit solutions if
there really are conflicts to solve.