Job posts are payable

This adds the basic functionality for payment and expiry of job posts. Job posts will update themselves every day with a background job (Heroku scheduler). Heroku scheduler can run rake tasks every day so it will run `update_posts` in scheduler.rake every day to hide expired posts.

## To do
[x] Add transactions table so we can store transactions properly.
[x] Add expiry dates to UI
[ ] Fix ugly "Display for 30 days" button