UI fixes and improvements

[j3ll3yfi5h]

Fixes https://github.com/statamic/cms/issues/3030 Fixes https://github.com/statamic/cms/issues/5034

• [x] introduce four main sections with z-indexes
• [x] position mainsections sticky instead absolute/fixed
• [x] prevent duplicate nav markup
• [x] improve table overflowing
  • [x] Collections
  • [x] Entries
  • [x] Terms
  • [x] Assets
  • [x] Fieldsets
  • [x] Forms
  • [x] Form Submissions
  • [x] Users
  • [x] User Groups
• [x] prevent popover overflows
• [x] change <header> children margins to flex gaps and fix overflowing headings

[jasonvarga]

To make this easier to review, I'd stop adding items to the scope of this PR.

Additional UI issues can be fixed in separate PRs.

If/when you're done with the items you've outlined in the description, go ahead and mark this as ready for review.

[j3ll3yfi5h]

Alright. I'm currently using the changes on a client project to see, how it behaves and I'll make it ready for review by the end of the day. 👍

[j3ll3yfi5h]

Oh, I did totally miss those fullscreen views, sorry!

I'll look into it, maybe there ist something we can do with the PortalVue.

[jackmcdade]

Or maybe it's mostly z-index? Previously each of those numbers had a purpose too (997, 998, etc) to account for the Bard Nav, main nav, etc etc.

[jackmcdade]

I also found that the table view gets way too tall

[j3ll3yfi5h]

I also found that the table view gets way too tall

This was in my opinion the nicest way to fix those popovers, which are causing scrollbars:

https://user-images.githubusercontent.com/6337411/189732667-e9b4386c-4748-4186-b299-05099bd09093.mov

At least ten fields in the filter without scrolling should work in my opinion - what is your opinion on this?

[jackmcdade]

Will be using this as inspiration for my work in #7559 — as it is right here I won't be able to merge it, but thank you for the hard work! It is most appreciated and valued.