Further CSS fixes

[mejo-]

📝 Summary

• Resolves: UX issues [Collectives 2.15]: Unreadable from mobile #1577
• Resolves: UX issue: Recent Pages section covering all the landing page on mobile #1578

Tested:

• Scrolling with long content on landing page with large desktop screen with/without search bar
• Scrolling with short content on landing page with large desktop screen with/without search bar
• Scrolling with long content on landing page with small mobile screen with/without search bar
• Scrolling with short content on landing page with small mobile screen with/without search bar

fix(css): New scroll container to fix page design

* Page title always sticky at the top
* Landing page widgets (optional) and page content in scroll container
* Search bar (if visible) always sticky at bottom

Also give the editor some minimum scroll height so that it's possible to
scroll down when the landing page widgets cover full page height (e.g.
on small mobile screens) and the page has no/short content.

fix(css): Fix maximum editor/page width on mobile
Maximum width should be the smaller value of either text editor width or
screen width.

🏁 Checklist

• Code is properly formatted (npm run lint / npm run stylelint / composer run cs:check)
• Sign-off message is added to all commits