Refactor @model/AbstractConditionValue to interface

[jenbutongit]

assumedly this is meant to be an interface rather than a class that should be subclassed

Originally posted by @jenbutongit in https://github.com/XGovFormBuilder/digital-form-builder/pull/179#discussion_r512639456

[endamccormack]

This needs more refinement and detail from the dev team.

P3

[endamccormack]

Just for readers' clarity. This issue is to replace the current ConditionValue abstraction class with an interface.