feat(ops/gerrit-tvl): init TVL Gerrit plugin

This is just going to be a grab bag of things which do TVL-specific
things to Gerrit, whether that be exposing new Prolog predicates or, as
I intend to do as the first thing, expose Buildkite builds as checks.

Change-Id: Iaeab987a1fdbd078b85e274691c986489903bf3a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2872
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

ops/gerrit-tvl/static/tvl.js

@@ -0,0 +1,3 @@
+Gerrit.install(plugin => {
+  console.log('TVL plugin initialising');
+});