cert_path now is api_cert_path
return nil as auto_ticket_path for invalid tickets

The auto_ticket_path is referenced from the tickets_controller like this:
     respond_with(@ticket, :location => auto_ticket_path(@ticket))
While this only uses the location parameter when the ticket is valid it will still attampt to calculate it if not. During the create action this will lead to crashes because the ticket_path can not be calculated for a ticket without an id.

This led to https://leap.se/code/issues/5552 and probably
https://leap.se/code/issues/5545.
let's only add the flash notice if the ticket has been created
check user_id param for present? instead of !nil?

when rendered from teh create action due to an error user_id param will
sometimes be an empty string. We should still avoid rendering the
navigation because the path's can not be resolved without a user_id.
add tests for invalid ticket creation
remove cert link - has no purpose anymore
Merge pull request #152 from azul/bugfix/5552-recover-from-invalid-tickets

Bugfix/5552 recover from invalid tickets