R1:ab4d33bede84

Allow underscores to appear in Harbormaster variable names

Summary:
See PHI859. Ref T13195. The regexp for replacing variables currently does not include underscores.

Include underscores.

I also made a note in T13088: we should (almost certainly?) throw immediately if you try to pass a bogus variable name as a custom parameter, but this is a slightly larger change.

Test Plan:
- Made an "http request" build plan with `?x=${initiator.phid}&y={$some_variable}`.
- Added `some_variable` as a parameter…