Only show course from latest year on home page

[jorg-vr]

This pull request removes old course cards from the home page, only showing the latest year. Using a dropdown it allows to cycle through previous years.

Favorited courses are always shown on top.

No dropdown is shown when there are less then two different years

Part of #2882

[bmesuere]

👍 Looks good, but I'm not sure about the current favorite behaviour. Shouldn't we add a separate "year" for favorites?
Two smaller things:

• Can you make the dropdown right-aligned (do the dropdown aligns with the right of the button?) Should be possible with <div class="dropdown-menu dropdown-menu-right">
• Can you style the years with an n-dash (?) as is done on the course cards?

[bmesuere]

👍 Looks good, but I'm not sure about the current favorite behaviour. Shouldn't we add a separate "year" for favorites?

• As an alternative, keep the current behaviour, but place the favorite courses in a separate "favorite courses" section above "courses".