fix: Fix group headers positionning

[marienfressinaud]

The previous hack didn't work as expected for the collections and feeds groups. After a certain number of groups, the header passed behind the cards.

There is still an issue on mobile: if you open the actions menu of a date group (not the first) and that you scroll up, the menu will appear behind the headers. That's however a bit less problematic. Also, I plan to change the popups behaviour by not fixing them to the screen.

Pull request checklist:

• code is manually tested
• interface works on both mobiles and big screens
• accessibility has been tested
• tests are updated
• French locale is synchronized
• documentation is updated (including comments, commit messages, migration notes, …)

If you think one of the item isn’t applicable to the PR, please check it
anyway and precise N/A next to it.