Instead of sending IQDB the image url and letting it download the file,
download the file on Danbooru's end and send IQDB the downloaded file.
This fixes several issues:
* Some sites need the referer header set to avoid hotlink protection
when downloading the file. Danbooru knows how to deal with this but
IQDB doesn't.
* We need to enforce certain restrictions when downloading files,
including setting max filesize limits, setting max timeouts, and not
allowing downloads from forbidden IPs (to avoid SSRF attacks).
Danbooru knows how to handle these things but IQDB doesn't.
* Fix the IQDB lookup to handle error messages returned by IQDB.
* Fix `undefined method `>=' for nil:NilClass` error when trying to upload
ugoiras. Using the preview image for IQDB lookups should be faster and
give the same results as the full image, plus it should work with things
like ugoiras and videos that IQDB can't handle.
Also add an image_url param so that API users can continue using the
full image if needed.
Previously the search form on the /iqdb_queries page submitted directly
to the iqdb service (karasuma.donmai.us), which redirected back to
Danbooru with the search results.
This was different than API requests, which submitted to
/iqdb_queries.json which proxied the call to iqdb through Danbooru.
Because of this, searches on the /iqdb_queries page had different
behavior than API requests. Things like filesize limits and referrer
spoofing were handled differently.
Now searches on the /iqdb_queries page submit directly to Danbooru. This
is simpler and it means that API requests and HTML requests have the
same behavior.
* Change cutoffs on upload page to max 5 results, min. 20% similarity.
* Change cutoffs on standalone /iqdb_queries page to max 20 results, min. 0% similarity.
* /iqdb_queries.json: add `limit` and `similarity` params to change default cutoffs.
