[API/TopicResource] Set thumbnail to None if background is None
Set background on test_topics.json
Refined background checks in TopicFactory to handle background deletion
Added specific api tests to check if background deletion works
Improved topic.form template (added thumbnail picture)
Cleaned unused code from EditTopicCtrl + added deleteTopicBackground scope & instance method
Cleaned Topic service code & improved update methods (see commit body)
+ added update method for field patching
+ overriden put method to avoid URL redirect (because of trailing /)