Due to the new Commentary request forms, which were
also used during vetting (choice=modify), the vetting
went wrong sometimes.
If the vetting action was 'modify', the user was redirected
to the old form which didn't exist anymore.

As a result of how the contruction of this modify structure, the Commentary
was first deleted and thus the data was lost. The requester did receive
a mail that his/her Commentary was accessible on SciPost however.

I fixed this by recontructing the vetting views. I combined the
vetting list and the vet request. I removed the action removing
the Commentary and made this a redirect to a new view which is
a modify page accessible only for EdCol Admins to modify+accept
the Commentary. Also, the mail for 'modify' vetting is also just sent
at *after* modification now.

Start transforming the prefill with Arxiv commentary request
 to a class based view using the Arxiv caller.

RequestCommentary is transformed to a class based view. Further
 moved strings to `/strings` and used django.messages middleware
 to pass messages to the user instead of context data.