Co-authored-by: Samantaz Fox <coding@samantaz.fr>

Co-authored-by: Samantaz Fox <coding@samantaz.fr>

Co-authored-by: Samantaz Fox <coding@samantaz.fr>

Rename CaptionsMetadata to Metadata
Nest Metadata under Captions
Unnest LANGUAGES constant from Metadata to main Captions module

PR #4037 introduced a workaround around YouTube's new integrity checks
on streaming URLs. However, the usage of this workaround prevents
storyboard data from being returned by InnerTube.

This commit fixes that by only using the workaround when calling try_fetch_streaming_data

An upstream problem with videojs-vtt-thumbnails means that URLs gets
joined incorrectly on any instance where `domain`, `external_port` and
`https_only` aren't set.

This commit adds some logic with the 404 handler to mitigate this
problem. This is however only a workaround.

See:
https://github.com/iv-org/invidious/issues/3117
https://github.com/chrisboustead/videojs-vtt-thumbnails/issues/31