Instead calling diff_collection.count use diff_collection.size which is cache on the diff_collection

Introducing the concept of SafeDiffs which relates 
diffs with UI highlighting.

1. Move render method to a concern, not a helper.
2. Let DiffHelper#diff_options automatically add the path option.
3. Move more instance var definitions to before filters.

When rendering a list of diff files, skip those where the diff is over
10 KB and provide an endpoint to render individually instead.

This reverts commit 13e37a3e.

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

Issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/12785
Merge Request: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3514

Closes #13467

This MR simplifies CI permission model:
- read_build: allows to read a list of builds, artifacts and trace
- update_build: allows to cancel and retry builds
- create_build: allows to create builds from gitlab-ci.yml (not yet implemented)
- admin_build: allows to manage triggers, runners and variables
- read_commit_status: allows to read a list of commit statuses (including the overall of builds)
- create_commit_status: allows to create a new commit status using API

Remove all extra methods to manage permission.
Made all controllers to use explicitly the new permissions.

Also fix a few places where page_404 should be render_404.

* Use commit objects instead of IDs when generating diffs
* Use proper references when generating MR's source and target
* Update broken specs

Now we apply syntax highlighting to the whole old and new files.
This basically help us to highlight adequately multiline content.