Work in progress. All unvetted theses are now listed at
theses/unvetted_thesislinks. Vetting editors click on
a thesis link to vet, and are redirected to
theses/vet_thesislink/<id>, where they can change fields of a thesis
link.

theses/vet_thesislink/<id> is now a regular update view. It does not have
mail capacity, or refusal reasons, etc. I will work on that next.

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.