Minor API tweaks

A few minor tweaks I needed while working on other things

* Minor improvements to Post/Form/Group/Attribute response array
  * Format created/updated timestamps in JS usable format
  * Don't return id+url array with relations if their empty
  * Return email/author/slug with posts
* FeatureContext: Move DB cleanup to seperate fn
* Additional validation /filtering for posts
  * Including auto generating post slugs
* Use built in ORM fn to check attribute key is unique (instead of custom function)