With this commit, we refactor the overall score handling of CodeOcean. Previously, "Score" and "Submit" were two distinct actions, requiring users to confirm the LTI transmission of their score (after assessing their submission). This yielded many questions and was unnecessary, since LTI parameters are no longer expiring after each use. Therefore, we can now transmit the current grade on each score run with the very same LTI parameters. As a consequence, the LTI consumer gets a more detailed history of the scores, enabling further analytical insights.
For users, the previous "Submit" button got replaced with a notification that is shown as soon as the full score got reached. Then, learners can decide to "finalize" their work on the given exercise, which will initiate a redirect to a follow-up action (as defined in the RedirectBehavior). This RedirectBehavior has also been unified and simplified for better readability.
As part of this refactoring, we rephrased the notifications and UX workflow of a) the LTI transmission, b) the finalization of an exercise (measured by reaching the full score) and c) the deadline handling (on time, within grace period, too late). Those information are now separately shown, potentially resulting in multiple notifications. As a side effect, they are much better maintainable, and the LTI transmission is more decoupled from this notification handling.
For the submission and comments, I mainly decided to use a `has_one` association. Based on the database schema, multiple request for comments could be allowed (i.e., for each file or submission), but this won't happen practically (since we always create new submissions and files). Hence, the `has_one` association is representing our relationship better.
* We want to identify a user that has triggered a testrun. Previously (in regular operation), only submission author who were regular users were able to start a testrun. Now, we want to prepare a future where submission authors are programming groups. Still, a testrun is triggered by an individual user and not a group.
* Further, this commit fixes some missing foreign key constrains.
