Updater (Swing): make it easy to initialize a personal update site

This change offers not only to initialize a personal update site given a
Fiji Wiki account and a password, but also to create a new Fiji Wiki
account (an email address is still required, and activating the account
using information sent to that email address, too, though).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>