Fix: Reduce z-index for some elements to prevent conflicts with embedded calendar popup

[Koc]

📝 Summary

Reproducer:

1. install Calendar module
2. embedd calendar into Collective
3. create event with huge text
4. open event in embedded calendar

🖼️ Screenshots

🏚️ Before	🏡 After

🏁 Checklist

• Code is properly formatted (npm run lint / npm run stylelint / composer run cs:check)
• Sign-off message is added to all commits
• Tests (unit, integration and/or end-to-end) passing and the changes are covered with tests
• Documentation (README or documentation) has been updated or is not required

[mejo-]

Thanks @Koc. While testing it I realized that the menubar in edit mode still overlays the calendar popup. And it seems like we set the z-index of 10021 for the menubar so it always is on top of the NcModal header, which has z-index: 10001.

From my manual tests I don't understand when the text menubar should overlay NcModal and wonder whether that's a leftover from the past. @max-nextcloud @juliusknorr do you have an idea?

[max-nextcloud]

@mejo- I do not remember. 83c1d21 Looks like the z-index it was there right from the start.

[Koc]

@mejo- thanx for finging, fixed

[mejo-]

After discussing this I think we should remove the z-index from page-info-bar, .text-menubar and .editor--outline altogether. For .page-title we should adjust it to only be higher than the editor content.

We need a respective PR in Text as well to do the same there. @Koc do you want to take care of this?