Consider building thumbnails from server as a fallback if user does not provide a thumbnail

[osdiab]

If for a video operation thumbnailUrl isn't present, we can generate it server side.

Rahul mentions that installing imagemagick on cloud functions is a pain, which is why we decided to do it on client. Ideally they should always both be present, but supporting this might make it more robust, for instance like now when apple doesn't accept our changes so iOS doesn't upload the thumbnail.

[rahulgi]

*i vaguely remember it seeming tricky to install imagemagick on appengine