Custom permalink structures require at least one valid structure tag, e.g. `%postname%`. If none is included, it would leave users with broken permalinks.
Let's make sure this won't happen by validating the permalink structure.
Adds unit tests.
Props rockwell15 for initial patch.
Fixes#35936.
git-svn-id: https://develop.svn.wordpress.org/trunk@37747 602fd350-edb4-49c9-b593-d223f7449a82