Fixed youtube thumbnails

API v2 got deprecated in March and v3 requires signup. Hotlinking like a
boss.
Added opt-in thumbnail proxying in upload

Rationale: some services thought that images linked from /posts/upload
are hotlinked. This is because they were receiving referrer set to a
host that is unknown for them. By using proxying, referrer is blank,
which increases thumbnail coverage at the expense of increased
server-side traffic.
Changed file layout to separate class
Added cache to upload thumbnails proxy