Creating a new ThesisLink didn't save the `requested_by`
 field, causing trouble in for example the vet view
 for these specific objects.

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.

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.

I keep get weird bugs. Maybe merging in development will help.

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.

The acknowledgement message is no longer hardcoded in
theses/views.py, but resides in the top-level strings module.

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

A CreateView class based view should replace the current
request_thesislink function based view, since it saves
many lines of code and is less error prone and more easily readable.