Merge branch 'new-sidebar' into 'master'

New sidebar design

I want to make separation between sidebar and content

- [x] change sidebar and header colors
- [x] change colors for old light theme so it matches dark style too
- [x] rename theme names to something more reasonable
- [x] fix header logo collapse with JS
- [x] fix navless layouts (explore page, login page, snippets, user page)
- [x] fix tests

See merge request !558