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.
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