cmd/web.go: check version after load config

- routers: use new binding convention to simplify code
- templates: able to set HTTP port number in install page

A new struct is created named EmailAddress that contains alternative
email addresses for users. Also the email related methods; IsEmailUsed
and GetUserByEmail are updated.

DeleteUser deletes the extra email addresses and DeleteInactivateUsers
also deletes inactive accounts. This could be factored out, but should
do it for now.

Just some code dedoublication in models/models.go