Issue 1382 - Improve patch viewing in Issues
    
        There are two issues here:
    
        [1] The current diff produced by html_diff() is lacking some important details.  Tabs and trailing white spaces are not correctly highlighted.  This commit creates a new pygment Style that is based off the current style "tango" and modifies it to use background color for changed lines for the html formatter which allows for the highlighting of trailing spaces.  Then a filter is added to the diff lexer that shows tabs, and allows trailing white spaces to be identified by the html formatter.
    
        [2] When opening an attachment in an "issue" check the file extension to see if its a patch file.  If it is then render the file in html to get our nice "diff" output.  This makes reviewing patches that are attached to Issues much easier to evaluate.
    
        Updated tests to account for the new html rendering of patches.
    
        Also fixed pep8 errors in filters.py & issues.py
    
        https://pagure.io/pagure/issue/1382