Matrix test mock

Create a test mock or server of the relevant Matrix APIs to aid in testing and development.

Th mock follows the same pattern as what was done for BBB: https://github.com/moodlehq/bigbluebutton_mock

See repository readme for instructions:

TODO:

•  [ ]Update and publish docker container (GA etc.)
•  [ ]Clean up repository (remove BBB implementation code)
•  [ ]Mock remaining API calls
  •  [ ]Add user to room
  •  [ ]remove user from room
  •  [ ]Upload media (partially done, needs testing and documentation)
  •  [ ]Change room avatar (needs testing and documentation)
  •  [ ]Get new access token from refresh token
•  [ ]Enforce uniqueness of `external_id` values
•  [ ]Add some existing authentication keys and/or a "backoffice" way to create them
•  [ ]Update documentation
•  [ ]Transfer to Moodle Github

References:

• Synapse API docs: https://matrix-org.github.io/synapse/latest/usage/administration/admin_api/index.html
• Matrix API docs: https://spec.matrix.org/v1.5/client-server-api/