[wip] Replace highlight.js with rouge gem

A few day ago I have opened this issue here #8404. I am neither a ruby nor a rails pro but I have managed to create some kind of demo. It is not ready for merge. You could consider it as a proof of concept and for comparing the results between `highlight.js` and [`rouge`](https://github.com/jneen/rouge)

## Again some explanations

* highlight.js is really buggy in getting the correct synthax. The solution is guessing the language with taking  the filename, shebang, ... into consideration.
* `pygments` handles this very well but it is a python library
* `rouge` is a highlighter in ruby which is compatible to pygments themes. It is able to do this kind of guessing, too.
* f4b9a65c05f24c3558687c98b5163b465dc46314 reactivates highlight.js language autodetection which is buggy by design.
* look here for some more discussion: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/232#note_622601, #8404

## TODO

- [x] Blob
- [x] Blame
- [x] Wiki
- [x] Code Blocks 
- [x] Snippets
- [x] Themes - find a better white and dark theme!
- [x] Check color definitions in themes again
- [ ] Code Cleanup
- [ ] Line number links
- [x] Line number colors
- [x] add new theme screenshots 

## Demo

Checkout my private Gitlab instance. I have included this pull request there already:
https://gitlab.sevenbyte.org/u/stefan

Instructions for changing the theme without an account are here: https://github.com/gitlabhq/gitlabhq/pull/8425#issuecomment-67137852
Unused method argument - `wrap`. If it's necessary, use `_` or `_wrap` as an argument name to indicate that it won't be used.