Bootstrapping an environment to quickly test the generated extension.
Created a job table with a lot of check constraints to ensure the job is valid
*when inserted*.

Also created security barrier views which can be used by regular users to scheduler their jobs.
Wrote a trigger which acts as a check constraint.
Removed the check constraints, as the data doesn't need to be valid in every context.

We will check on insert or update whether the context for the job is correct.
It will raise some sane error messages if a user does something they shouldn't.
Refactoring of files: moved validation functions and trigger for job table into its own file
Added more validation,
wrote error message which should provide enough details for the user scheduling a job.
Talking with Oleksii about some changes.

Documented them in DESIGN.md
Merge pull request #1 from zalando/data_model

Data model, design considerations