Set the default User-Agent sent for outgoing http requests.

* Add a `Danbooru.config.http_headers` config option.

* Refactor various places to use this option instead of setting the user
  agent manually.

test/helpers/download_helper.rb

@@ -14,7 +14,7 @@ module DownloadTestHelper
     tempfile = Tempfile.new("danbooru-test")
     download = Downloads::File.new(test_source, tempfile.path)
 
-    rewritten_source, headers, _ = download.before_download(test_source, {}, {})
+    rewritten_source, _, _ = download.before_download(test_source, {})
     assert_equal(expected_source, rewritten_source, "Tested source URL: #{test_source}")
   end