searchable: add framework for defining user search permissions.

Add a `visible_for_search` method to ApplicationPolicy that lets us
define which fields a user is allowed to search for.

For example, when a normal user searches for post flags by flagger name,
they're only allowed to see their own flags, not flags by other users.
But when a mod searches for flags by flagger name, they're allowed to
see all flags, except for flags on their own uploads.

This framework lets us define these rules in the `visible_for_search`
method in the model's policy class, rather than as special cases in the
`search` method of each model.

app/policies/note_version_policy.rb

@@ -0,0 +1,4 @@
+# frozen_string_literal: true
+
+class NoteVersionPolicy < ApplicationPolicy
+end