Reverse Merge -  Ooops, this only works in EF6!

Refactored RockCSharpMigrationCodeGenerator to limit Migration operations to only Entities that are directly associated with the DbContext, either as a DbSet<> of the DbContext or as just an AssociationTable (from relationship definitions in the DbContext) (reverse-merged from commit cfd030449484e83e605053657e6cea69f3ecf92c)