Write tests and refactor html.

Fix bug that allows a forged post request to create a comment,
while the submission is not open for commenting.

Only works for comments on thesis links. All comment forms
should post to this link from now on! (So all logic pertaining
comment creation should disappear from the models' detail view.)

I also added a `new_comment.html` partial template and added tests.

The Comment model now has a option to
 append a file (pdf, png, jpg) to itself.

Improve comments app to follow PEP rules
 and remove all wildcard imports!!