Redacted: use a FL token when available

- Use a FL token when available
- Added a configuration (default off) to enable the feature
- Skip prefetching the data with Deluge as with Transmission in
`preprocess()`

- Note: because QBittorrent & uTorrent need the data to compute the
torrenthash but still require the url to download the file, it will
result in 2 FL tokens used instead of one because the .torrent will be
downloaded twice from the server.

Fixes #3163

data/interfaces/default/config.html

@@ -686,6 +686,10 @@
                                             <label>Seed Ratio</label>
                                             <input type="text" class="override-float" name="redacted_ratio" value="${config['redacted_ratio']}" size="10" title="Stop seeding when ratio met, 0 = unlimited. Scheduled job will remove torrent when post processed and finished seeding">
                                         </div>
+                                        <div class="row">
+                                            <label>Use freeleech tokens when available</label>
+                                            <input type="checkbox" name="redacted_use_fltoken" value="1" ${config['redacted_use_fltoken']}>
+                                        </div>
                                     </div>
                                 </fieldset>