Remove HeraldActionConfig, HeraldFieldConfig

Summary:
Ref T2769. Move all of this stuff into Adapters and get rid of the hard-coded classes.

I cheated in two places.

Test Plan: Edited and activated Herald rules.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, chad

Maniphest Tasks: T2769

Differential Revision: https://secure.phabricator.com/D6688