We first introduced `policy_violations` as a means to store data for the
Policy evaluations, and at the time we took the opportunity to include
more data in the table than was present previously.

Over time, we've seen that the new `advisories` (which replaced
`custom_advisories`) table is necessary to follow the same shape of
data, including pulling in data from each of the tables to provide a
single view of all advisories.

However, since we've made `advisories` use the same shape of data, it's
unreasonable to have both tables, when there's no purpose and it leads
to confusion.

As part of this, we can also close off some subtle implementation bugs
that were due to the complexity of the two tables having a fair bit of
overlap.

This was largely due to the way that we were allowed to use non-`POLICY`
advisory types, so lookups across tables were a little awkward.

Closes #452, #298, #400, #340.