Ytdl-sub

[BACKEND] Remove old format for `video_tags`, `music_tags`, `download_strategy` (899)

Now that sufficient time has passed, we can formally deprecate the following:
- https://ytdl-sub.readthedocs.io/en/latest/deprecation_notices.html#video-tags
- https://ytdl-sub.readthedocs.io/en/latest/deprecation_notices.html#music-tags
- usage of `download_strategy` (simply remove it if you're getting an error about it)

[FEATURE] Toggleable plugin field `enable` for all dict-based plugins (897)

For key/value-based plugins, add an `enable` field to make it easier for child presets to disable their functionality

[BUGFIX] Smarter subscription validation (895)

With the right functions, it was possible for subscription validation to raise a false-positive error. This should hopefully resolve that issue and give validation a small performance increase

[BUGFIX] Fix rare case when yt-dlp returns None and is downloaded (894)

[BUGFIX] keep_max_files=0 downloading 2 (893)

Fixes https://github.com/jmbannon/ytdl-sub/issues/892

Does not set max_downloads if keep_max_files = 0

[BUGFIX] Only apply --dl-override to subscriptions in --match (890)

As title, to not accidently add breaking changes for subscriptions that are not going to run