We need to support migration of several different types of legacy reference fields to D8:
- Node and user reference, for both D6 and D7
- D7 taxonomy term reference fields
- D7 entity reference fields
Create a base class for cckfield plugins that handle all reference-type fields, since they tend to be more similar than they are different.
Write the patch, tests, docs. Let's pull ReferenceBase out of the patchzilla in #191, clean it up, and turn it into its own patch.
User interface changes