Add functionality to edit comment in a PR
    
    With this commit, we can now edit comment on the PR. To accomplish this
    we have added new endpoints in ui/fork.py and made changes in
    templates/_formhelper.html. This commits also adds a file
    pull_request_comment_update.html which is a form for editing comments.
    Function for the same functionality is added in pagure/lib/__init__.py.
    
    We also added two new fields in pull_request_comments table in DB:
    editor_id, edited_on. Corresponding changes in _formheper.html are included to display 'last edited by' in the UI.