Move seeds into a mix task to avoid further issues with breaking them

[begedin]

Problem

Moving our seeds script into a mix task would allow us to

• have it fail on serious compilation issues
• write a simple test which makes sure the task runs without errors

It could also be the first step towards creating a seeds context, though the necessity of that part is worth a discussion first.

[joshsmith]

Will need an estimate before deciding how to prioritize this.