chore: add more advanced policy configuration (#9726)

This codifies a review policy which is closer to what I always
envisioned, but which isn't expressible using the normal checks in the
GitHub GUI. It would move the commit approval check from GitHub into the
policy-bot check which is already present to enforce the
conventional-commits standard. Approvals in general would still work the
same -- it's just that the bot picks it up and toggles the status
accordingly. From a GitHub side when this is enabled we'd remove the
requires-review check from there and let the bot decide that part. We
would still require builds and tests to pass of course.

There are a couple of relexations from the current policy, details in
the code but briefly:

- Changes to translations or dependencies by a trusted person don't
require review
- Trivial changes by a trusted person, explicitly marked as such, don't
require review

This enables less bureaucracy for things like adding new translated
languages and updating dependencies, and enables the trivial-change
workflow to a larger audience than, like, me, who could always just
bypass the rules by way of being admin.

.github/CODEOWNERS

@@ -1,2 +0,0 @@
-/AUTHORS    @calmh
-/*.md       @calmh