Support registering a new account via this client.

Support registering a new account via this client.

This is provided as a second workflow on the TUI login page. You get
to enter a username, email address and password, and register your
account. You still have to respond to an email sent by the
server (just as much as logging in, it needs to know that you're
really the owner of that email address); this can be done by clicking
the link in the message, or pasting it into the login workflow,
whichever you think is easiest.

Clicking the link has weird effects if you set a redirect_uri of
"urn:ietf:wg:oauth:2.0:oob": the Mastodon web server actually tries to
redirect your browser to that URI, which causes at least Firefox to
prompt for an application to open it in, which is thoroughly confusing
and looks more like an error message than the indication of success it
actually is. So instead I've made it redirect to a page on my web
site, which is still a bit odd, but less odd than that.