Redirect '/prefs/wikiaccount' to '/prefs' if the user has an associated account

(or their account is __error__, in which case they still can't do
anything on that page)

In any case, they never see a link to /prefs/wikiaccount after their
account has been associated or marked __error__.
Fix links to the wiki's user create pages in email templates

[#61261016]
Whitespace fixup.
Mention that they'll be creating a wiki account on the registration page

[#61261016]