models: Centralize generation of filenames

models: Centralize generation of filenames

Move filename generation to a mixin. This allows us to reuse the code
for other items like cover letters. Some unncessary 'strip' calls are
removed as their unnecessary.

This allows us to change the file extension for diffs to 'diff', which
is a little more accurate.

Signed-off-by: Stephen Finucane <stephen@that.guru>