Add initial support for Kobo device Sync endpoint.

- Supports /v1/library/sync call to get list of books
- Supports /v1/library/metadata call to get metadata for a given book
  + Assumes books are stored on Backblaze for metadata call
- Changes to helper.py so that we can return no cover instead of a blank
image.

cps/templates/config_edit.html

@@ -104,6 +104,10 @@
                     <!--option-- value="3" {% if config.config_updatechannel == 3 %}selected{% endif %}>{{_('Nightly (Automatic)')}}</option-->
             </select>
         </div>
+        <div class="form-group">
+          <label for="config_server_url">{{_('Server Url. This is only used for the (experimental) Kobo device library sync')}}</label>
+          <input type="text" class="form-control" name="config_server_url" id="config_server_url" value="{% if config.config_server_url != None %}{{ config.config_server_url }}{% endif %}" autocomplete="off">
+        </div>
       </div>
     </div>
   </div>