Resolve Merge conflicts in editor

## What does this MR do?

This is the second iteration of https://gitlab.com/gitlab-org/gitlab-ce/issues/20344 which now allows to resolve conflicts manually providing an editor to the user.

## Are there points in the code the reviewer needs to double check?

- Previous implementation has been refactored almost from the ground up.
- Also some components like `parallel-conflict-lines` has been optimized to work well with PhantomJS.
- The app and files required files are loaded only when needed. `merge_conflicts_bundle.js` is only loaded in `/<group>/<project>/merge_requests/<Id>/conflicts` only

## Why was this MR needed?

Some MRs were unable to be solved interactively in the UI, with this MR we let the user to manually fix complex merge request conflicts.

## Screenshots (if relevant)

<img src="/uploads/33a016f025bd590b1fc6eeee1ee11626/Screen_Shot_2016-09-19_at_1.39.39_PM.png" width="800">

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/20344, https://gitlab.com/gitlab-org/gitlab-ce/issues/3567.

See merge request !6374