[LTI] Allow a single per-course activity

[tbarbette]

Is your feature request related to a problem? Please describe.
When using many Inginious tasks in Moodle, we have to configure and copy-paste by hand a dozen fields for each activity. It's very cumbersome.

Describe the solution you'd like
Simplest A) Have a per-course activity instead of per-task.

When creating an instance of the LTI tool on Moodle (or any LMS), the first view can simply let me select one INGInious task that is the one I want to use in this activity. Then afterwards it shows the right INGInious task directly. I think Gradescope kind of does that.

The LTI exposes enough course information and activity id to create a unique mapping between the unique activity and the selected task.

This is simpler because currently the INGInious LTI is organised course-wide. We could even maybe have another URL (using / instead of /task for instance) to enable both the old way and course-wide way.

Alternative B) Moodle-wide INGinious integration.

The advantage of having simply a global, INGInious-wide integration is that a Moodle administrator (eg Moodle UCLouvain) might add the activity for everybody, from there it would help foster the use of INGInious. But that part requires much more changes as the LTI keys etc would need to be INGInious-wide.

From a user point of view, when creating an activity I would have to select one of my courses and then one of my tasks, it's not much more complicated.

Describe alternatives you've considered
Ask assistants to do it.

Additional context
Screenshot of the pain: