Automatic subtitles: mark them as automatically generated

[JohnXLivingston]

Describe the problem to be solved

When subtitles are generated with the new feature coming with v6.2.0-RC1, they should be marked as "automatically generated".

Describe the solution you would like

For example, Youtube differentiates automatic subtitles and user added subtitles by adding the information in the subtitle name (see screenshot, in french):

For the record, there is also this suggestion that could be implemented:
https://ideas.joinpeertube.org/posts/230/allow-multiple-subtitles-for-the-same-language

[Chocobozzz]

Thanks for the suggestion! Implemented in 5d62c3c