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.
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Adds information about changes in the current release, links to changlogs from
previous releases, and improves links to use `https`.
Release date for the latest versions will be August 18 even in the event of a
delayed core release. Since all themes are backwards compatible we don't have
a dependency there.
Props mercime, davidakennedy.
Fixes#32978.
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