Don't present users with Herald fields/actions for uninstalled applications, unless the rule already uses them

Summary:
Fixes T7961. Currently, we present Herald users with actions like "Require legalpad signatures" and "Run build plans" even if Legalpad and Harbormaster are not installed.

Instead, allow fields and actions to be made "unavailable", which means that we won't present them as options when adding to new or existing rules.

If you edit a rule which already uses one of these fields or actions, it isn't affected.

Test Plan:
  - Created a rule with a legalpad action, uninstalled legalpad, edited the rule. Action remained untouched.
  - Created a new rule, wasn't offered the legalpad action.
  - Reinstalled the application, saw the action again.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T7961

Differential Revision: https://secure.phabricator.com/D20808