Support LTI 1.3 - Githubissues

This issue contains the to-do list for LTI 1.3 support.

[x] Add fields to theLTILaunch model linking to LTI_11_ResourceLink or LTI_13_ResourceLink objects, instead of just to a resource, so we can tell which link a user clicked on.
[x] Create a deep-linking flow. You should be asked to either pick an existing resource or create a new one, and then it should create an LTI_13_ResourceLink and return to the platform.
[x] Use the Names and Roles Provisioning Service to get a list of students for the access changes form.
[x] Look at whether the assignment and grades service could improve how reporting scores is handled.
[x] Tidy up which views expect there to be LTI data, and give good error messages when it's not present.
[x] Check that LTI 1.1 still works.
[x] Now that middleware handles the LTI launch data, trim out the bits of CachedLTI_13_Mixin that fetch the same data.
Could we use custom launch parameters to specify things like access changes, and let the platform determine who has access to those links? Under what circumstances would you want to have two links to the same resource?
Use a prefix to the URL for the LTI launch ID, instead of a GET parameter? Then the reverse_with_lti function would just have to stick a prefix in front of the URL, instead of parsing and unparsing the query string.

numbas / numbas-lti-provider