Rethink the student dashboard

[Splines]

Here, we redesign the entire student dashboard and integrate with the new Müsli-related features. The dashboard has two sections: "You are registered for these" and "You bookmarked these". The semester that the students want to view can be chosen from a dropdown, which is repeated above the search bar.

Note that currently the dashboard is also the same for tutors, teachers and admins. This will change in follow-up PRs, they will get their own variants/additions to this dashboard.

Tip

Please use Flipper.disable(:next_term_banner) as we don't need this banner anymore (and will remove it in a future PR).

Tip

This PR comes with new seed data such that we can actually see all scenarios on the dashboard. This seed data is not yet on the mampf-init-data repo. Instead run rails seeds:build on the dashboard branch after having freshly seeded.

Lecture Cards

The lecture cards themselves have colored washi tapes and the color for them can be modified by users (and is stored in the DB). They get quick action buttons (in the form of "speech bubbles") that inform about upcoming assignments, new forum posts and new comments. Lecture cards also show a progress bar for assignments (once the first assignment lies in the past). Lecture cards are sorted according to confirmed / pending / rejected registration.

Warning

Some quick action buttons will currently bring you to the course outline of a lecture instead of its lecture dashboard. This is not an issue of this PR, and will be tackled when we redesign the lecture dashboard itself.
Furthermore, this PR does not touch the "subscription model" in the lecture dashboard itself:

"You have not subscribed to this lecture yet. Subscribing gives you access to its materials — it is not a binding course registration.".

While the button is renamed to "bookmark", this is not actually what we want in the long run. Instead, no content should be gated behind a request to subscribe/bookmark a lecture (only when password is required). See a future PR.

Lecture Cards of the Search

The lecture cards of the search get new action buttons and and indicators to bookmark or register for a lecture/talk, or to see that a registration is pending or rejected.