Remove thumbnail encoding wherever possible and store as files

Summary

Currently we store thumbnails as a thumbnail encoding on both ContentNodes and Channels.

This is problematic when we come to publish channels because we have to create thumbnail files for all of the encoded thumbnails.

It is not as simple as just create files and discarding the thumbnail file object, as frequently thumbnails are uploaded 'whole' and then cropped to create a thumbnail. We need to retain both copies to allow later editing.

Suggested solution:

A new file preset: THUMBNAIL_SOURCE that is applied to the uploaded original file, while a THUMBNAIL preset is created to represent the actually currently used thumbnail (this is also to remove the current disaggregation of the thumbnail presets by kind).

A similar mechanism should be done for the Channel thumbnail also, with the thumbnail files being attached to the root node of the channel main tree.

Two caveats:

• an encoding will still need to be generated at publish time to send as part of the public channels endpoint for Kolibri and in Kolibri channel databases
• A migration will need to happen to update all current data to accord to this new pattern - this can happen in a background job, and then we can remove the legacy code that handles the encoding

Start by mapping the existing thumbnail encoding paths for ContentNodes and Channels, including how channel thumbnails attach to the root node. The work is complete when source and current thumbnail files use the proposed presets, publish-time encodings still work, and existing data has been migrated without the legacy encoding path.