API versioning

[rixx]

This PR is part of #1598 and introduces versioned API capabilities to pretalx, while making sure that nothing will change for users of the current and next release.

• Get serializers based on API version
• Get API version from request (by header), but only allow LEGACY and DEV
• Implement token model
  • token should have only read-only vs read-write permissions for now
  • possibly optional event limit?
• migrate tokens
• UI for token management: create, delete. no modifying permissions after the fact, I think
• Get API version from token
• Test: errors
• Log use of deprecated endpoints
• Document API versioning in writable-api-wip branch (otherwise people reading the docs would think the feature is ready for prod …)