Herald actions: Allow setting subtype on Maniphest tasks

Summary:
https://secure.phabricator.com/T12314 introduced task subtypes. Allow Herald rules which change/set the subtype of a task.
Code originally written by @20after4 for Wikimedia.

Closes T16022

Test Plan:

1. Have the default subtype configuration with three types under http://phorge.localhost/config/edit/maniphest.subtypes/ defined in src/applications/maniphest/config/PhabricatorManiphestConfigOptions.php
2. Go to http://phorge.localhost/herald/edit/?content_type=HeraldManiphestTaskAdapter&rule_type=global
3. Under "Action", select the new "Change subtype to" option. Test the subtype search by clicking the magnifier icon, set up a Herald rule to test execution.
4. Remove the config for http://phorge.localhost/config/edit/maniphest.subtypes/
5. Repeat step 3, no explosions, default "Task" subtype still exists after removing the config.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, 20after4, Cigaryno

Maniphest Tasks: T16022

Differential Revision: https://we.phorge.it/D25913