Rich workspace polishing

[juliushaertl]

While trying to quickly implement #5018 I noticed a few quirks around rich workspaces and ended up digging deeper, so this is a combination of richworkspace fixes for 28

• fix: Avoid having multiple rich workspace instances active
  • I noticed that text sessions from richworkspaces where still continuing to issue requests after switching folders. It turned out that the F2V way of rendering the headers will call render on every folder change, but not on empty folders. Therefore we cannot reuse an already created vue instance for the rich workspace and need to make sure to destroy it properly before rendering a new one.
• fix: Keep track of local state of the rich workspace separately
  • If felt cleaner to not reuse the showRichWorkspace/hideRichWorkspace methods since they are explicitly for toggling the setting, so i use a local state that is initialized with the prop value but may be changed afterwards through creating/deleting/renaming a README file
• fix: Make loading of rich workspace pixel perfect
  • The read only editor has a global server css margin which caused a minor 3px jump, now with this commit the switch between preview and text session is not noticable
• fix: Remove minimum height to only use needed space
  • Actual fix I wanted to do 🙈

📝 Summary

• Resolves: Rich workspaces takes too much screen estate when Readme.md is empty #5018

[juliushaertl]

/backport to stable28