Numerous small UI fixes:
- Fix double border in collaborator list
- Fix system notice table background
- Mute links in repo and org lists
- Downsize projects edit buttons
- Improve milestones and project list rendering
- Condense milestone list entry to a single line of "metas"
- Mute ".." button in repo files list
Various fixes to pages or elements which were looking ugly on mobile.
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Co-authored by @silverwind
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Co-authored-by: silverwind <me@silverwind.io>