Job expiry date

[darylyu]

This PR has multiple changes:

• Changes Job.expiry to Job.expiry_date. This also changes the field type from DateTimeField to DateTimeField. This will make it easier to run a scheduled daily task that marks jobs as expired or disabled. This also makes querying easier as we probably won't care about the specific hour and time.
• Moves get_default_expiry to jobsboard.common.utils from jobsboard.jobs.models.
• Changes the default for Job.employment_type to EMPLOYMENT_TYPE_FULL_TIME from EMPLOYMENT_TYPE_PART_TIME as most job posts look for full-time employees instead of part-time employees.
• [x] Design
• [x] PEP8 for Python
• [x] Standard for JavaScript

[darylyu]

If accepted, this PR affects the specification in https://github.com/pythonph/jobs-board/issues/4