Show more relevant info for teachers on course cards

[jorg-vr]

This pull request updates the course cards from #4261 , so that they display more relevant information for teachers.

The course progress is replaced by some generals statistics:

• the number of users
• the number of series
• the number of exercises
• the number of submissions

The content of the card displays the following info (In order of priority, with max three being shown):

• the number of unanswered questions, with a link to the questions page
• the number of pending users, with a link to review them
• the number of submissions that still need to be reviewed, with a link to the first of those submissions
• Up to 1 series with a recently passed deadline (within last week)
• Up to 2 series with an upcoming deadline
• Up to 3 series which have recently been worked on by a the most users

The series progress bars are replaced by class progress. This visualizes the amount of users that have completed a certain amount of exercises.
This class progress is now also used for series displayed in the top cards on the homepage.

The icons for open_for_institution are also removed as this is the default and does not require extra attention of the course_admin.

I have also added a progress bar on the homepage when switching between academic year. (And slightly improved it's styling because the blue on pink clashed)

Visually this looks like:

• Tests were added

Part of #2882

[jorg-vr]

Mockup 1 (WIP):

• Added overview statistics
• Added Action items for pending registration and open questions
  

todo:

Add statistics that can scale from one item to full card. Either:

• current activity
• class progress
• ?
  First explore statistics on course page without space limitations

Fix heights

Fix open/closed icon for registration

[bmesuere]

I would leave out the number of questions from the header (but definitely include it in the card itself if there would be any). The other numbers could be abbreviated to for example 15.5K instead of 15484.

The priority of things to show are:

• open questions
• pending registrations
• series with upcoming deadline
• series with passed deadline

For now, I would fill the rest of the slots with exercises (which is not really useful). In a later phase/PR, we could add some graphs instead of exercises.

For courses using series without deadlines, could we show something instead of the graphs? Maybe "series being worked on now"?

[jorg-vr]

[niknetniko]

This looks very good! (I have added some small nitpicks here and there; an automated ERB formatter would be very nice)

One small side-effect I noticed though is that the home page now takes a bit longer to render. This is normal, but when using the academic year selector, there is no indication that something is actually happening. So for example, if I select another year, it feels like nothing is happening, which made me think something went wrong (although it was just loading). I think we should either add a loader or just go for a full page refresh (in which case the browser shows something is happening).