R1:d60d4e6a0521

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…