Modernize meeting page css

[elpoelma]

Overview

This PR slightly modernizes the meeting page css by moving to a flex-based layout.
In terms of UX/UI the behaviour should have stayed more-or-less the same.

connected issues and PRs:

None

Setup

None

How to test/reproduce

• Login as a municipality and open/create a meeting
• Both the behaviour of the sidebar and main content should have stayed the same:
  • Both containers scroll independently
  • Paddings have remained the same/similar
  • The page is still more-or-less responsive: the sidebar disappears when reducing the width of the page + the main section takes full width

Checks PR readiness

• UI: works on smaller screen sizes
• UI: feedback for any loading/error states
• Check cancel/go-back flows
• Check database state correct when deleting/updating (especially regarding relationships)
• changelog
• npm lint
• no new deprecations