Default Themes: Update version numbers and readme files for 4.7 release

Bump version numbers and update readme files for themes with changes. Also updates description for Twenty Seventeen and removes version number for HTML5 shiv for easier updates in the future.

Props davidakennedy.

Fixes #38858.

Built from https://develop.svn.wordpress.org/trunk@39496


git-svn-id: https://core.svn.wordpress.org/trunk@39436 1a063a9b-81f0-0310-95a4-ce76da25c4cd