I18N: Add translator context to all 'Preview' strings.

[itzmekhokan]

https://core.trac.wordpress.org/ticket/64986
Adds translator context to all in-tree occurrences of the 'Preview' string using a simplified _x( 'Preview', 'noun' ) or _x( 'Preview', 'verb' ), so translators can correctly disambiguate it across languages (e.g. German) where the noun and verb forms differ.

• Buttons and action links use 'verb'.
• Preview pane labels, table row headers, and the iframe title use 'noun'.

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