Require an "open for pull requests" issue label on PRs

Require an "open for pull requests" issue label on PRs

Replace the pull request template with the new contribution policy: a pull
request must reference an issue in this repo that is open and carries the
"open for pull requests" label, or it is closed automatically.

The rule is deliberately without exceptions, including for documentation and
typo fixes, which the previous template explicitly waved through.  A rule with
exceptions is a rule that gets argued about on every pull request that lands
near the boundary, and the argument costs more than labeling a two line issue
does.  CONTRIBUTING describes the process and what each of the issue labels
means for a contributor.

The issue templates gain a note pointing at the same policy, so the
expectation is set when the issue is opened rather than when the pull request
is closed.

The gate is implemented as a github webhook handler in publishthing
(publishthing.apps.prgate), configured per project from build_ln.

Change-Id: Id24e6542b9953b2916e7654ee9859932d4af1bff