[berkerpeksag]

Refs #591

@malemburg I didn't remove django_comments_xtd completely. Job posters and reviewers can still see old comments, but I'm planning to remove the whole app after a deprecation period.

I've also fixed some small bugs in email templates.

TODOs

• Add more tests

[berkerpeksag]

Ok, I think this is finally ready to go. Here's a summary of this PR:

• Made django_comments_xtd read-only. I'm going to open another issue to migrate old comments into jobs.JobReviewComment.
• Wrote a lot of tests to make sure that I didn't break anything. Numbers of the most important files of the jobs app:

  jobs/forms.py                                         38      0      4      0   100%
  jobs/managers.py                                      39      2      0      0    95%
  jobs/models.py                                       131      5     18      6    93%
  jobs/views.py                                        218      6     32      4    95%
  

• Fixed a lot of issues regarding to email templates. See the commits for details.
• Deleted unnecessary code and simplified get_queryset() and get_object() logic in jobs/views.py.
• Made several UX improvements. See the commits for details.