Refactored tumblr subscription creation view to use common mixin

Major improvements:  recipient names are shown, default recipient is first in list, can't select "---' anymore.

Tumblr's existence is validated much earlier in the form-saving process, allowing us to report the error directly to the user (that tumblr doesn't exist!) as a form error.

Tumblrs can be specified via url (http://demo.tumblr.com), url-without-protocol (demo.tumblr.com) or just the short name.

Exception handling for tumblr 404s also improved.