Refuse to create an OSGeo UserID associated with the email address of an already existing user

[strk]

In order to reduce possible email abuse we should forbid the creation of multiple users associated with the same email. This is to allow for email-based recognition of fake/malicious users, for example.

Also, users that create a new account with the same email might be just missing a "password reset" procedure instead, so it would be better to propose an alternative.

[strk]

When refusing to create such an account, a message should clearly state how to contact an administrator, as with the mantra-based registration in its current incarnation.

[strk]

Fixed: ​https://git.osgeo.org/gogs/sac/web-cgi-bin/commit/43ca5918decf4b0faba471647fe14de84951c52d