Remove the `preview_urls` method from strategies. The only place this was used was
when doing IQDB searches, to download the thumbnail image from the source instead of
the full image.
This wasn't worth it for a few reasons:
* Thumbnails on other sites are sometimes not the size we want, which could affect
IQDB results.
* Grabbing thumbnails is complex for some sites. You can't always just rewrite the
image URL. Sometimes it requires extra API calls, which can be slower than just
grabbing the full image.
* For videos and animations, thumbnails from other sites don't always match our
thumbnails. We do smart thumbnail generation to try to avoid blank thumbnails, which
means we don't always pick the first frame, which could affect IQDB results.
API changes:
* /iqdb_queries?search[file_url] now downloads the URL as is without any modification.
Before it tried to change thumbnail and sample size image URLs to the full version.
* /iqdb_queries?search[url] now returns an error if the URL is for a HTML page that
contains multiple images. Before it would grab only the first image and silently
ignore the rest.
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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