Migrate to new Pretalx API

[NMertsch]

Summary

Pretalx updated their API and it broke our setup.
This PR fixes it, the YouTube integration was not tested (my token cannot access /p/youtube, so I used make all EXCLUDE=youtube)

Fixes #132

Details

API Changes

Nested resources

In the old API, nested resources were always embedded in the parent response body:

GET /speakers/<ID>
{
  ...
  answers: [
    {"id:" ..., "question": {...}, ...},
    ...
  ]
}

In the new API, nested resources are only referenced by their IDs:

GET /speakers/<ID>
{
  ...
  answers: [
    123,
    ...
  ]
}

The old behavior can be restored with the expand query:

GET /speakers/<ID>?expand=answers,answers.question
{
  ...
  answers: [
    {"id:" ..., "question": {...}, ...},
    ...
  ]
}

This PR adds expand queries where necessary.

Removed query ?questions=all

Previously, we used ?questions=all on the /submissions and speakers/ endpoints. This is not supported anymore. Answers and questions are now handled like all other nested resources (see above).

This PR removes these queries.

Schedule slots

Previously, the response to /schedule contained the keys slots and breaks:

• slots contained all submission slots (talks, tutorials, ...) and contained objects of type "Submission"
• breaks contained all non-submission slots (lunch and coffee breaks) and contained objects of another type
• Both these object types contained the key slot with the slot details (time, location).

Previously, the responses to /submissions contained one slot object per submission (the first slot).

In the new version, /schedule only returns slots. The response objects contain the slot details (time, location), and an optional reference to the submission.
The responses to /submissions now contain slots (all slots) instead of slot (first slot).

This PR uses pydantic's BeforeValidators to re-arrange the data to the old format before deserialization.

Changed structure of localized names

Before:

GET /submissions/<ID>
{
  "track": {"en": "<NAME>"}
}

After:

GET /submissions/<ID>
{
  "track": {"name": {"en": "<NAME>"}}
}

This PR updates the deserialization logic for submission.track, submission.submission_type, and slot.room.

Renamed speaker.avatar to speaker.avatar_url

This PR updates the deserialization keyword, but keeps avatar in the transformation result.

[egeakman]

Looks good! Thank you so much Niklas!