This allows us to take advantage of Rails' `to_partial_path` to render
the correct partial based on the Blob type, rather than cluttering the
view with conditionals.

It also allows (and will allow in the future) better encapsulation for
Blob-related logic which makes sense for our Rails app but might not
make as much sense for the core `gitlab_git` library, such as detecting
if the blob is an SVG.

Closes https://github.com/gitlabhq/gitlabhq/issues/9265

* It is more performant given now we process all the diff file instead
  of processing line by line.
* Multiline comments are highlighted correctly.

This custom formatter for rouge is needed to generate HTML output
specifically for gitlab. Since its usecase is mostly suitable for
gitlab it had been rejected upstream:
https://github.com/jneen/rouge/pull/268

Thanks a lot to @stanhu, @jneen and @tsigo for review!

I have mainly created the rugments fork for the purpose of improving
gitlab's highlighting. Nowadays IMO it works way better than the old
highlight.js solution. But the development is stuck on my side because
of a couple of personal reasons:

  * I have finished my studies; last months I was writing my master
    thesis. So there was a huge time problem. I am sorry for that.

  * I had to move to Munich due to getting a (paid) job. Searching a
    flat here is horrible... :)

  * Last but not least, maintaining the same code base in two seperate
    projects is a mess.

I have decided to switch back to rouge due to several reasons:

  * In the beginning I was quite motivated, but since I start
    working on my new job next week, the best solution IMO is
    switching back to upstream rouge.

  * Rouge is continously improving:
    https://github.com/jneen/rouge/blob/master/CHANGELOG.md
    http://rouge.jneen.net/

  * There should be absolutely no regressions with this change. Most
    likely this pull request will almost fix some minor bugs.

  * One less gem in gitlab is a good thing. since Gitlab is quite a
    huge bundle of gems. Reducing complexity should be a major
    milestone.

Thanks a lot to @stanhu and @jneen for the review!

Closes #1521

if something goes wrong, fall back to the plaintext lexer

Make the following changes to deal with new behavior in Rails 4.1.2:

* Use nested resources to avoid slashes in arguments to path helpers.

I decided to create a fork of rouge as rouge lacks a HTML formatter with
the required options such as wrapping a line with <span> tags.
Furthermore I was not really convinced about the clarity of rouge's
source code.

Rugments 1.0.0beta3 for now only includes some basic linting and a new
HTML formatter. Everything else should behave the same.

nohighlight functionality for a hardcoded set of filenames is kept

Because I am tired of CHANGELOG highlighted as sql file :)
Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>