I18N: Add translator context to disambiguate translation strings for "Preview".

This changeset disambiguates translation strings for "Preview" by adding a verb|noun context to its occurrences.

Props timse201, audrasjb, anupkankale, mohamedahamed, rafaeldella, motylanogha, wildworks, jorbin, khokansardar, sergeybiryukov.
Fixes #64986.


git-svn-id: https://develop.svn.wordpress.org/trunk@62353 602fd350-edb4-49c9-b593-d223f7449a82