About page: Add strings for 4.7 and make available for translation.

It doesn't look very pretty; design changes still to come.

props helen, michaelarestad, samuelsidler, jbpaul17, jorbin, nacin, melchoyce.
see #38616.

Built from https://develop.svn.wordpress.org/trunk@39420


git-svn-id: https://core.svn.wordpress.org/trunk@39360 1a063a9b-81f0-0310-95a4-ce76da25c4cd