Try to merge into develop branch ASAP, then continue minor fixes afterwards.

Closes #3182

* render code view server side

* remove debug print

* fix multiline selection bug

* change string concatenation to bytes.Buffer for efficiency

* Fix newlines added by previous for hljs

* fix selection highlighting

* make css changes in .less

* Lighter icon colors for repo files

* also color submodule icon

- Update language choose dropdown to fit more languages

- add border-radius to filter list items
- use color as background of label for issue/pull view page

By adding a Japanese font in the less file, easier to see the display in
Japanese.

Both are possible on explore and admin panel

Collaborators have write access as default, and can be changed via repository
collaboration settings page to change between read, write and admin.