Fix: Fixed some warnings in API import cron job
Enhancement: Added CREST link generator

Added a CREST link generator for mails with sufficient context information
- the external ID, the victim's external ID, the final blow pilot's
  external ID, the destroyed ship and the timestamp.
If this information is present the CREST link for a kill will be exposed
on the kill's detail page.
The generated CREST hash will NOT be updated in the database, it's
generated on the fly.