Add private jobs flag to jobs

• Added a migration file to add is_private field to jobs with default value being false.
• Added a not_private scope to job.rb and chained it withstatus_on and Job.pending so that status pages does not include private jobs.
• Chanined not_private scope with Job.history so that package history pages do not include private jobs.
• Added is_private: false to query in GET /:user/jobs/? if unauthorized user or job history is not of logged in user so that private jobs are visible to users who created them.
• Added/Updated tests in spec/debci for the above.