Meta Tags should use a MediaField

[lawrencedine]

Ideally the MetaTagsPart should be using a MediaField directly. This would prevent issues like #28 from happening.

Consideration would have to be given for the migration process from storing the image URLs on the part directly and it would probably be a good idea to maintain that property as a facade over the new Field to keep existing projects happy.

[peterkeating]

Another example why we should be using fields for this - #41