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.
This directory contains library code used through Danbooru. This includes things like defining API clients, dealing with
sources, parsing tag searches, storing and resizing images, and so on.
Many of the files here use the Service Object pattern. Instead of putting complex code in models or controllers, it goes
here, in plain old Ruby objects (POROs). This keeps models and controllers simpler, and keeps domain logic isolated and
independent from the database and the HTTP request cycle.
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