Add edit button to MergeRequest diff

It will redirect you to file edit page.
After submit changes you will be redirected back to merge request.

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Add changelog entry

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Add style guidelines in separte paragraph.
Solve conflict and add more explanations on how to contribute.

Conflicts:
	CONTRIBUTING.md
Merge branch 'master' into styleguide

Conflicts:
	CONTRIBUTING.md
Markdown doesn't render correctly yet.
 Merge branch 'styleguide' into 'master'

Styleguide

As discussed.
Merge branch 'feature/edit_from_mr'

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Conflicts:
	CHANGELOG
Merge branch 'master' of dev.gitlab.org:gitlab/gitlabhq