I added a RequestHandler that uses Nicknym::Source - but that has not
been implemented yet.

Whenever a RequestHandler class is called we instantiate it with the request.
Then we call handle on the instance. This way we can access the request and
its content via accessors rather than only in the handle method.

So far we would error out if no host was specified in the config or
the request. It's true that we can't do local lookup if we don't
know our own domain. However we can still use HKP.

In the future we will query leaps own API for other providers. If the
host was not set in the initial request we might even proxy a request to
ourselves. Providing the Host header will prevent an infinite loop in
that case.

InvalidEmailHandler - handle emails with an invalid format
LocalEmailHandler   - handle emails on the local domain
EmailHandler        - handle all other emails by using hkp

This is a preparation to add leap provider email lookup and remove
hkp eventually. But for now we keep the behaviour the same and only
refactor.