Provide enough data to call H from a modified LTI launch view

We intend to expose an authenticated end-point which will provide as much information as is required to make sensible use of the H API using an OAuth 1 signed LTI launch as the authentication mechanism and API.

This end-point will:

* Accept standard LTI params to identify the user and context etc.
* Return JSON
* Contain the auth token and URL to call H
* Contain a listing of groupings related to the present context (either course or assignment level)
* (Maybe?) Provide a mapping of LMS user ids to H user ids
* Identify the present user

### Implementation notes

This will probably be a real pain to test as we will need to be able to generate valid signed requests (which the LMS usually does for us), and could be a good candidate for functional tests.

### Tasks

* https://github.com/hypothesis/lms/pull/4205 - Proof of concept
* [x] https://github.com/hypothesis/lms/pull/4222
* [x] https://github.com/hypothesis/lms/pull/4224
* [x] https://github.com/hypothesis/lms/pull/4230
* [x] https://github.com/hypothesis/lms/pull/4295
* [ ] https://github.com/hypothesis/lms/pull/4307
* [ ] https://github.com/hypothesis/h/pull/7555
* [ ] https://github.com/hypothesis/h/pull/7556
* [ ] https://github.com/hypothesis/h/pull/7557

Maybe we don't need:
* [ ] https://github.com/hypothesis/lms/pull/4272
* [ ] Add user information to the return values
* [ ] Add role information to the return values