This prevents situation when user registered 2nd level domain and then
tries to register a subdomain. In such case i2p router doesn't know yet
this 2nd level domain and cannot access it. Using b32 address avoids
such situation.
When there is verification error and "reason" is present, no reply was
returned to client causing django exception:
ValueError: The view pyi2phosts.postkey.views.subdomain didn't return an
HttpResponse object.
If user wants to register a subdomain, he must prove he is owner of
2nd-level domain. He must create a file with random name generated by us
in webserver's document root in 2nd-level domain. Then we trying to
fetch this file. If it success, registration gets accepted.
