Refactor the pull_request.html template
    
    Previously, the pull_request.html template was used for 3 different
    pages: the PR page, the create PR page(displayed as a diff of two
    branches when not logged in), and diffing two commits.
    
    This PR pulls the actual rendering of the diff out into a new macro
    file, that is shared between three new templates:
    
    * repo_pull_request.html
    * repo_compare_commits.html
    * repo_new_pull_request.html
    
    This does not change any of the style of the pages, it is simply
    an abstraction of the templates to make the upcoming restyling
    a lot easier.
    
    Signed-off-by: Ryan Lerch <rlerch@redhat.com></rlerch@redhat.com>