Refactor `canvas_helpers.py` to split out `add_thumbnail` functionality

[digitaldogsbody]

I believe @helrond already suggested this at one point, but it would be good to split out the helpers in canvas_helpers.py into their own classes (and also probably files?), which would allow us to target the functionality more explicitly.

For example, many other items can have a thumbnail property, and so the add_thumbnail() helper could be very useful for things like Collections or Manifests, but at the minute it is only patched onto Canvas, and it currently cannot be patched onto any other classes without also giving them the add_image helper, which would be invalid in most (all?) other cases.