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

- Update language choose dropdown to fit more languages

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.

- copy link button: #1396, #1168, #1668,
- synxtax highlight: #1712, #1549, #1315, #670
- z-index: #1942