Mighty GitHub PRs

This is an extension to restyle Pull Request screen for easier navigation and more focused review sessions in GitHub (public or any private/enterprise edition).

Main features:
- works on corporate enterprise domains, you just need to enable in the context menu of the extension in Chrome toolbar:
- shows file tree on the left hand side
- folders are collapsible
- folders are as concise as possible (if a folder containing on single folder it is merged with parent in visualization)
- deleted files are red and crossed through in file tree
- seen files become non-highlighted once seen
- only one single file's diff is shown if a file is selected from the tree
- large, visible Approve and Needs work buttons added ('Need work' button automatically adds a generic 'Please fix review items' comment.)
- clutter is reduced - elements on the top of the page is eliminated in the file navigation view to eliminate visual noise