Tech Debt: Make WPAccount.defaultBlog a lazy getter rather than relation

[aerych]

Follow up on work started in #12704 to refactor WPAccount defaultBlog to a lazy getter rather than a relation.

[aerych]

I've self-assigned this one but if someone wants to tackle it before I find the time feel free. :)
cc @designsimply in case this one makes sense for groundskeeping.

[elibud]

@aerych should we remove your assignment here?

[dangermattic]

Thanks for reporting! 👍

[jkmassel]

This is probably a good idea, but we're not going to do it because we're replacing Core Data.

Closing.