Introduced FieldData

FieldData is an object nested inside an Entry's content.
Most implementation was on the client side, there's no FieldDataSchema in use right now because mongoose does not supported subdocuments outside of an Array. Having all FieldData in an array felt bad, what with not being able to access the data easily.

Every FieldData has a default `value` field inside its `data` object. This will contain default values. Future field types may populate the `data` object even further. Appropriate methods have been created.

User templates can still use `{{body}}` to access the value for a body. When the Markdown plugin middleware is finished, templates can also start using `{{body_html}}`.