Improve data shown on home page

[bmesuere]

The homepage currently is quite static and could do more to surface relevant information. Especially for users that are only subscribed to a single course, the home page is relatively empty. This could be improved by adding a row of "action cards" to the top of the home page:

• The leftmost card is "jump back in" and is shown in blue. This shows a recent exercise that wasn't yet correctly solved. If there are no such recent exercises, the card could lead to a recent incomplete series. Clicking on the card brings the user to the respective exercise, potentially with their last submission already loaded in the editor.
• Next, a number of relevant "deadline" cards are shown. These cards are sourced from all courses a student is subscribed to. The color of the card depends if the series status. It is green if the user successfully solved all exercises and red otherwise. For teachers, it could indicate how many students already completed the series.
• A maximum of 4 cards is shown.

Todo

• Include a marker/icon (bottom right?) to indicate the cards are clickable.
• Include a progress bar and more status information in the deadline cards. We do have to make sure all of the card types have the same height.

Open questions

• Should we always show these cards, or only on desktop screens?
• Are there other cards/actions we could show here?
• Can this replace part of the right sidebar?
• I'm not sure about the style of the action cards. An alternative could be to have white cards (with or without drop shadow) with a colored (more saturated) border instead.

Current mockup

The html of this mockup can be found in 6120e84.

[bmesuere]

Duplicate of #2882