Original PR description

We add a button to the existing tax report "Connect / Send"
to HMRC in order to send the tax information to the government.

The first time you connect, you have to log in into the hmrc site
first.  For that, we needed a proxy on our site to recuperate its
response (as they support only a fixed redirect uri).  The proxy
will check if a valid db uuid is given.  The proxy itself generates
a token and you get a token back from hmrc.  Every 4 hours, the token
needs to be refreshed.

We use two endpoints of the API: retrieving the obligations (when
and for which period vat returns (still) need to be filled in) and
submitting the vat return itself.

When you click the send button after a successfull login, a wizard
is loaded where if possible the obligation is selected which conforms
with the dates you chose in the report.  There you can send it.