Merge tag '1flow-0.28.1.7' into develop

0.28.1.7 1flow-0.28.1.7
Allow to put subscriptions back to “un-sorted” pseudo-folder.
version bump for 0.28.1.8.
Merge tag '1flow-0.28.1.8' into develop

0.28.1.8 1flow-0.28.1.8
Merge branch 'release/0.28.1.8'
Add a default value to Feed.thumbnail_url to avoid http://dev.1flow.net/1flow/1flow/group/30453/
version bump for 0.28.1.9.
Merge branch 'release/0.28.1.9'