Fix password storage

[cgay]

Still storing passwords in cleartext. I meant to fix this when I was originally working on the git back-end code but forgot. (Hooray for not writing stuff down.)

Also, whatever mechanism is used to encode the password must not contain colons, given the storage format.

Also (but not closely related), put some verbiage on the registration page saying not to use secure passwords on this site.

[cgay]

Or rather, to not use passwords used elsewhere.