Make signup error messages work when USERENA_ACTIVATION_REQUIRED is false

Checking whether a username / email address is in use by an active
or unconfirmed account (for purposes of showing a helpful error
message) should take into account the value of
USERENA_ACTIVATION_REQUIRED: if this is False then any conflicting
attempt will incorrectly receive the 'This username is already taken...'
message as there will not be any associated UserenaSignup objects.

Fixes #321