Refactor and Decouple Seeds

[toastercup]

Currently, seed tasks are happening in all the wrong places in Cortex. We should abstract most of them to our engines, and rebuild them using DSLs or with a helper library like Seedbank

Of particular interest is the seed for Employer Blogs. This is not a concern of core nor our plugins-core engine. We should create an entirely new engine for this, or figure out a better way of handling default structures - perhaps this belongs in the parent application of core?

[MKwenhua]

I am ok either way.