Post migration should return linked media

Problem/Motivation

Right now, there is no way to link the post with its associated media within the set of migrations in the module. This relation is identified by the `post_parent` column on the `posts` table and this could be inversed for the posts migration (similar to what we already do for terms).

Steps to reproduce

N/A

Proposed resolution

Add source property called `post_media` that contains all media IDs (post IDs)

Remaining tasks

Do it

User interface changes

None

API changes

Adding a source property on the WpPost migration

Data model changes

None