Change structure of Lisk for custom setters and getters

Summary:
This diff changes the way Lisk should be used for custom setters and getters,
changing it from having subclasses of Lisk implement their custom setter or
getter to having them override the readField and writeField methods (which get
called by the getters and setters). This diff also has a configurable option
to throw an exception if a subclass of Lisk implements a custom setter or
getter.

Test Plan:
Without the config set to throw, tested in sandbox by browsing differential
and playing with the differential typeahead. With the config set to throw,
tried to load a phabricator page and saw in the error log an exception thrown
by Lisk because of custom getters in PhabricatorUser.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, jungejason, epriestley

Differential Revision: 974