thumbnails in LBRY

Nobody has claimed this yet.

the goal is to store claim thumbnails within the LBRY network

### changes

- every command that takes `thumbnail_url` also takes `thumbnail_path`, a path on the local filesystem to the thumbnail image
- if given, the sdk turns that file into a stream and puts the sd hash into the thumbnail Source. it also makes the file available on the DHT

there might be some complexity in the file manager or stream manager because we used to have the assumption that one claim = one stream. but with this, one claim might have two streams (the stream itself and the thumbnail)

### open questions

- there's no way to store image dimensions. is that ok?
- what filetypes are acceptable for thumbnails?
- how to return thumbnail data back to the app?
- will displaying a list of claims be slow because you have to download all the thumbnails?

Start with the commands that accept thumbnail_url, then trace the file manager, stream manager, and DHT handling described in the issue. Determine how thumbnail_path should become a second stream and how thumbnail data returns to the app. The open questions about dimensions, file types, and list performance must be resolved before completion can be defined.

Written by the indexing model from the issue text.