On Harbormaster build plans, show which Herald rules trigger builds

Summary:
Ref T13258. Provide an easy way to find rules which trigger a particular build plan from the build plan page.

The implementation here ends up a little messy: we can't just search for actionType = 'build' AND targetPHID = '<build plan PHID>' since the field is a blob of JSON.

Instead, make rules indexable and write a "build plan is affected by rule actions" edge when indexing rules, then search on that edge.

For now, only "Run Build Plan: ..." rules actually write this edge, since I think (?) that it doesn't really have meaningful values for other edge types today. Maybe "Call Webhooks", and you could get a link from a hook to rules that trigger it? Reasonable to do in the future.

Things end up a little bit rough overall, but I think this panel is pretty useful to add to the Build Plan page.

This index needs to be rebuilt with bin/search index --type HeraldRule. I'll call this out in the changelog but I'm not planning to explicitly migrate or add an activity, since this is only really important for larger installs and they probably (?) read the changelog. As rules are edited over time, this will converge to the right behavior even if you don't rebuild the index.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13258

Differential Revision: https://secure.phabricator.com/D20259