adding a thumbnail does not enable the 'publish' button

[indirectlylit]

Summary

Reported by @lyw07 in learningequality/kolibri#3821:

After adding a thumbnail to the channel (originally the channel doesn't have a thumbnail), I'm not able to publish the channel directly. So I had to add a new file to the channel and publish it

Category

BUG

How to reproduce

• create a channel with no thumbnail
• add a file
• publish
• add a thumbnail

At this point you should be able to re-publish

Real-life consequences (anything community should be aware of, for instance how it affects end users)

Users cannot publish meta-data changes to their channels without making "dummy" changes