Test if thesis is vetted/deleted and mail is sent in
the accepted/refused case.

Theses that are accepted are vetted by the correct contributor.
Mails are not yet sent. Thesis is not yet deleted when refused.

This does not work yet. I wrote some failing tests, but while
I was doing that, I came to the conclusion that I want to make
the route of the get request different from the route of the post
request.

Removes some 40 lines of boilerplate, since CreateView
handles the creation of a new ThesisLink.