Add reminder that playlist is private in share modal

[MyNameIsTroll]

Describe the problem to be solved
I have little experience with playlists on Peertube.
By default, when I create a playlist it is in "private" mode.
A few days later, I try to share my playlist, I press the "share" button, I get a sharing link.
Except that my playlist is in "private" mode... so if I share this link my friends will see a 404 error.

Describe the solution you would like:
2 things could be improved in my opinion.

The first is that we could warn the user when he presses the button to create a sharing link that his playlist is in "private" mode and that this link will only be visible to him. he is connected to his account.
Otherwise, invite him to change the state of his playlist from private mode to public or unlisted mode.

Second, if it continues its process anyway and shares a "private" playlist link and someone follows the link and lands on the 404 error page, we might be able to mention in the errors possible on page 404 "this may be a private mode playlist".

[natar10]

Hey! Do you still need help with this issue?

[Chocobozzz]

@natar10 yes, you can create a PR if you want

[natar10]

Hello there! I opened the PR with a possible solution here

Please let me know any comments. Thank you!

[Chocobozzz]

Fixed by #4469