Improve working with multiple tabs in CodeOcean (as learner)

[MrSerth]

Currently, working with multiple browser tabs for a single exercise is not well supported by CodeOcean. This is caused by the way we handle changes to the ACE editor and how we save submissions (fully replacing any old saved state). In the past, this has caused some learners to loose their progress, when a second tab with the initial exercise state was changed (and thus saved consequentially).

Potentially, we can simply activate the editor synchronization added for the pair programming feature, so that all editors are always in sync.

Furthermore, we should check support for different LTI sessions, caused by opening multiple exercises in parallel. Historically, this meant that only the exercise opened the latest could be submitted, but this might have changed. Still, the login workflow is still expected to invalidate the CSRF token and thus cause ActionController::InvalidAuthenticityToken exceptions (handled with a 422, currently).