Entry field references

[perry-mitchell]

Add ability to reference fields from other entries. The value would appear when displayed, but not be stored internally. For instance: Entry A may have a field called test, and Entry B may reference A's test field. When A's test is changed, rendering B will show the new value for field test.

Areas that this will need to affect:

• Entry#getProperty (return value)
• Entry#setProperty (set remote value)
• Entry#toObject (include referenced values)
• Entry facades (get and set)
• Entry create reference
• Entry remove reference (option: copy value or remove field)

Referencing an Entry property should work like so:

• An attribute is set on the Entry, flagging that a property is linked
• The value of the property on this Entry will be set to the ID of the entry it is referencing