Close #51

- Make `Host.is_editable`, `Show.is_editable`, `Note.is_editable`, and
  `Note.get_audio_url` static methods,
- Make `Schedule.instantiate_upcoming`, `Schedule.generate_timeslots`,
  `Schedule.get_collisions`, `Schedule.generate_conflicts`,
  `Schedule.make_conflicts` and `Schedule.resolve_conflicts` static
  methods. Most of these methods do not belong here.
- Fix signatures of methods for subclasses of `viewsets.ModelViewSet`
- Fix doctrins for subclasses of `viewsets.ModelViewSet`
- Fix comparisons with None (replace '==' and '!=' with 'is' and 'is not')

Also clean-up unused imports.

- Field "FundingCategory" is not mandatory anymore
- Added new field "is_active": Active shows will be determined either by existing timeslots in the future (if show is active). If show is not active it will always be considered as inactive, even if there are upcoming timeslots.

Fixes #25

+
* Fixed bug when deleting a note
* Extended schedule list view and filters in admin ui
* Got rid of unnecessary scopes
* Rearranged settings

* for adding a number of (business) days to the schedule (e.g. "on the rrules' following day")
* fixed a bug for monthly recurrences
* and some minor changes

See #8

* added corsheaders middleware to work properly
* fixed bug when updating a timeslot
* after updating a timeslot, its repetition will be returned if there's one
* included field 'ppoi' in shows

See #22 #23

See #21 #23

and:
* Set correct login redirect url for oidc
* Added django-cors-headers for cross-domain api requests

See #21 #22

* Properly generate thumbnails for shows, hosts and notes
* Return thumbnails in API

See #22

...and filters for admin ui and api.

See #21

* Users can see and change all notes and hosts of shows they own - even if they were created by another show owner
* A host can be assigned to a note (if host is editable)
* Added django-oidc-provider app
* Added limit/offset pagination to endpoints /shows, /timeslots and /notes
* Common users may not change the show's name anymore via API
* Playout API can be called with start (date) and end (date) parameters
* Added filters to endpoints:

/shows/?host=1  Retrieves shows of a given host
/shows/?owner=1 Retrieves shows of a given owner (= user ID)
/notes/?host=1  Retrieves notes assigned to a host
/notes/?owner=1 Retrieves notes a user has created (= user ID)

Had to set settings const USE_TZ (= reflect timezone in mysql datetimes) to True for OIDC to work with mysql.

Don't know yet if that's a problem.

See #22 #23

See #21

/api/v1/shows/
/api/v1/shows/?active=true
/api/v1/shows/1/
/api/v1/shows/1/schedules
/api/v1/shows/1/schedules/1
/api/v1/shows/1/schedules/1/timeslots
/api/v1/shows/1/schedules/1/timeslots/1/
/api/v1/shows/1/schedules/1/timeslots/1/notes
/api/v1/shows/1/schedules/1/timeslots/1/notes/1
/api/v1/shows/1/notes
/api/v1/shows/1/notes/1
/api/v1/shows/1/timeslots
/api/v1/shows/1/timeslots/1
/api/v1/shows/1/timeslots/1/notes
/api/v1/shows/1/timeslots/1/notes/1

On every /timeslot endpoint, filter variables ?start=2017-01-01&end=2017-12-31 are available.

Properly create, update and delete:

* Users
  - only superusers may create
  - common users can only retrieve/update themselves
  - update is constrained to certain non-crucial fields for non-superusers

* Shows
  - only superusers may create
  - update is constrained to certain fields for non-superusers

* Notes
  - creating/updating is constrained to shows and timeslots the non-superuser owns

* Schedules
  - only superusers may add/update/delete
  - collision detection still needed

* Timeslots
  - direct creation is prohibited for everyone (only by adding/updating schedules)
  - update is constrained to certain fields
  - only superusers may delete


Available GET endpoints for (only superusers may add/update/delete)

* Hosts
* Types
* Topics
* Categories
* RTRCategories
* Languages


See #22 #23

Problem to solve: there's a mix between API usecases for Playlist-UI and Website.

See #22

Shall we still put this in a sub app?
Would still need to figure out how to hook from the sub app into the serializer viewsets (and not vice versa) in order the custom fields get saved.

See #21

Only Admins (if is_staff is True) may access these Models for reading/writing.

Linking notes to timeslots also uses the REST API now.

Adding/Updating/Deleting schedules including collision detection is still a to-do.

See #22

Updating is constrained to defined fields.
See #22

Implemented django-oauth-toolkit for authentication.

Working on prototype for playout API (momentarily still called week_schedule())

Added permission restrictions for common users in APIs
See #23

Added playlist_id to TimeSlot datamodel
See #21

Made some minor changes in calendar.