Add a report for policy violations

Now we've produced policy violations in #258 and #268, we should also
make them easily queryable.

We can also expose this in a new report, `policy-violations`, which
allows us to view these as-is, which may be important for certain users
interacting with DMD.

As we want to allow filtering on the CLI and the web, we should allow
tweaking the parameters, requiring we have a separate query for
performing `Like`s.

This also provides the wiring for both the CLI's reporting, and for
`dmd-web`.

Closes #275.