Update the jobs table to add real reference to users table

• Added migration file to add requestor_id field containing the id of user requesting the job, making the field foreign key for users and deleting the existing string requestor field.
• Added association belongs_to in job.rb for user with validation of presence of user and has_many in user.rb for jobs.
• Updated other files to use requestor association with user object
• Updated tests in spec/debci files.
• Added tests in spec/debci/job_spec.rb to check null value constraint data integrity constraint with foreign key validation.