Call it 'Distraction-free writing mode' and update the help string.

Merges [30890] to the 4.1 branch.

Fixes #30657.

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Finalize two more about page strings.

Merges [30887] to the 4.1 branch.

See #30435

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Updates to the 'Log out everywhere' implementation.
 * Include a message and a disabled button when you're only logged in at one location.
 * Avoid leaking the session token in HTML.
 * Simplify, simplify, simplify.

Merges [30888] to the 4.1 branch.

Fixes #30264.

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Update/add plugin recommendation strings.

Update [30887], add a translation function to the Vine string.

Merges [30889] to the 4.1 branch.

Fixes #30337.

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Autoprefixer for [30857].

Merges [30884] to the 4.1 branch.

See #30435.

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