Add created_at timestamp for new services

Per issue #9
Sort services UI by creation timestamp

At some point in the future, it would be nice to also show the creation
timestamp. Given that most services don't have one at the moment, we'll
leave that alone for now.

Fixes #9
Add numericality validation to created_at field

Because yay data quality.
Show creation timestamp in services/show.erb

Per feedback in #19
Merge pull request #19 from ausaccessfed/feature/created-at-timestamp

Add 'created at' timestamp to Rapid Connect services