GitLab

Frontend for Merge Request Diff

This merge request improves the UX for the merge request diff feature which was recently implemented here (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6127). Specifically, it styles various parts of the diff feature to match the designs, it disables comment-related buttons in states where comments are disabled, and it adds a 'Show latest version' button for convenience.

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

I could use feedback on this MR's fidelity to the design.  

## Why was this MR needed?

Neccessary styling improvements for basic UX of this feature, and enabled comment buttons are not functional and thus need to be disabled in certain states.

## Screenshots (if relevant)

![57dd0755f0b14342305909](/uploads/318a44a3bc8b7fc5c9c6ef92ba92e511/57dd0755f0b14342305909.gif)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab....