Facebook plugin: Handle FB Oct-2013 Likes changes

Rather than a count on posts, likes are presented as a paginated array.
Added test for new style and normalize both presentations into a standard
format during stream processing.

Closes #1717, closes #1760
Update the instructions on creating a new database migration.
Merge pull request #1768 from cdmoyer/Updated-new-db-migration-instructions

Docs: Update the instructions on creating a new database migration [ci skip]