feat: set media player preference as enabled by default

[ayoayco]

Proposal to promote the media player as not experimental 😄

We decided to put this feature as experimental to gather feedback and bug reports. I looked through the issues to see if there are complaints regarding the media player, but there seems to be none! 🎉

This PR will

• set the preference to show embedded media player as true by default
• promote embedded media player as not experimental

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[userquin]

Restore old experimental value and delete it once the new preference saved (the problem, we don't know if the user has disabled the experimental feature so we need to restore the original value if experimental still there, otherwise we don't need to change the new feature since it is enabled by default)

/cc @patak-dev @shuuji3

[ayoayco]

I will check, thanks!!

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[shuuji3]

Since setting this as default means we think it's stable and recommend using it, I think we could let this one time change and merge it as is? (For me, I've been using this nice player for a while and haven't seen any issues.)

Some people might have tried it and turned it off, but I think there are many more people who haven't even tried it yet. Keeping the existing value will let those people miss this useful feature after that.

[shuuji3]

The current default value is false (I think it was automatically stored after update) so we cannot distinguish if the value false means that it was turned off manually.