allow users to subscribe to comments w/o registering

allow users to subscribe to comments w/o registering

Technically, when the user does this, a passwordless account is created
for them. The notify mails include a login url, and once logged in that
way, the user can enter a password to get a regular account (although
one with an annoying username).

This all requires the passwordauth plugin is enabled. A future enhancement
could be to split the passwordless user concept out into a separate plugin.