Settings: flip local video feed by default

[jbtrystram]

Most (if not all) video chat apps show my local video mirrored. Element not doing it by default is makes it confusing at first. Even my phone camera app does it.

Not only matching other video chat tools out there, 99% of times you see yourself in your day to day life is through a mirror, which flips the image. So matching that makes the most sense, to preserve continuity.

This setting was made optional in #5437 but not defaulted to true.

Fixes #10651

Checklist

• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• I have licensed the changes to Element by completing the Contributor License Agreement (CLA)

[yuguen-intel]

LGTM

[dbkr]

There was some contention on this when it was introduced so this should probably be a product decision. That said, this will only be for legacy 1:1 calls anyway, and EC mirrors the local video feed with no option (although is smart enough to only do it for front-facing ones).

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