feat: adds a publishing system to validate job post submissions

[aaronarduino]

Hello! I decided to give a go at adding post validation to help cut down on spam submissions(#50). My thinking was that it might be better to create a published flag in the db rather than a validation flag since that would allow better semantics around validating email addresses in the future. This would require job posts to be published before they go live.

Things changed:

• Renamed published_at db column to created_at in db.
• Added a published bool and published_at nullable timestamp cols to db.
• On job creation set created at timestamp in db.
• Update email verbiage to include language about job created in draft state and the need to edit and publish.
• On edit page update wording of Update button to Publish.
• When Publish on the edit page is clicked update published col to true and published_at to current timestamp, then save job.
• Update index route to filter out unpublished job posts.

Let me know if you think anything should be improved or changed. Thanks!