Store API IDs alongside LTI ones for users and courses

[marcospri]

LMS always have two separate IDs for the same course and user, one relevant for LTI (launches, grading APIs...) and one for the proprietary LMS API (groups, files integration).

Until now we have only stored the LTI ones in the DB and rely on custom launch parameters to obtain the API ones. This becomes a problem to interact with the API outside the context of a launch.