googlephotos: create feature/favorites directory - Fixes #4189

Enable access “Favorite” images on Google Photos backend.

This adds a “feature/favorites” folder in the Google Photos backend
and uses the Feature Filter API:

https://developers.google.com/photos/library/reference/rest/v1/mediaItems/search#Filters

docs/content/googlephotos.md

@@ -173,6 +173,10 @@ into albums.
 - shared-album
     - album name
     - album name/sub
+- feature
+    - favorites
+        - file1.jpg
+        - file2.jpg
 ```
 
 There are two writable parts of the tree, the `upload` directory and