Mighty GitHub PRs per Adam Graf

Main features:
- works on corporate enterprise domains, you just need to enable in the context menu of the extension in Mozilla Firefox toolbar:
- file tree on the left hand side (with collapsible width)
- 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
- 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