Misc UI fixes, add secondary color (#13378)
* Misc UI fixes, add secondary color
- Add secondary color, primarily used in arc-green currently
- Convert icons on release page to SVG
- Improve resolved conversation placeholder
- Diff fixes on arc-green
- Misc color tweaks
* fix comment header, adjust arc-green dropzone
* label margin, sidebar margin
* flexbox commits table and add primary button styles
* tooltip styles
* file header fixes
Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: zeripath <art27@cantab.net>
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