Followup: Sidebar in spaces-management

[lookacat]

Followup issues from #8192

• "details" panel fine tuning (Show space default image image and hide member count in spaces management #8236)
  • open right sidebar, click on space "A", number of members (below the cover image) doesn't update when clicking on space "B", "C", etc. (@JammingBen )
  • cover image is shown if I'm a member of the space. cover image should never be shown anywhere in the spaces view of the admin-settings app (@JammingBen )
• the multi selected space panel could show some more info. E.g. how many spaces are selected and what their combined quota values are (total, remaining, used). (@lookacat) ([full-ci] Extend multiple space selection details #8230)
• Actions-panel: Available actions:
  • Rename
  • Edit subtitle
  • edit quota
  • disable/enable
  • delete (if disabled)
• Members-panel (Admin panel members list #8224)
  • show members in read-only mode
  • dont show existing links.
• Use the SpaceNoSelection component in the files-app (@lookacat )
• Add filter to members-panel (Add filter to the space member panel in space management #8228)
• Fix display on smaller screens when sidebar is opened (@lookacat)
• add a header section to sub-panels which shows the space name (same as for the folder or file name in the files app when navigating into sub-panels) (Add header to the sidebar in spaces management #8234)
• Unit tests