Separate reading object values out of didSetObject() in CustomField

Summary:
Ref T3886. Broadly, fields break down into two types right now: fields which store data on the object (like `DifferentialTitleField`) and fields which store data in custom field storage.

The former type generally reads data from the object into local storage prior to editing, then writes it back afterward. Currently, this happens in `didSetObject()`.

However, now that we load and set objects from ApplicationTransactionQuery, we'll do this extra read-field-values on view interfaces too. There, it's unnecessary and sometimes throws data-attached exceptions.

Instead, separate these concepts, and do all the read-from-object / read-from-storage in one logical chunk, separate from `didSetObject()`.

Test Plan:
  - Edited Differential revision.
  - Edited Maniphest task.
  - Edited Project.
  - Edited user profile.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3886

Differential Revision: https://secure.phabricator.com/D8299