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.
Built from https://develop.svn.wordpress.org/trunk@37747
git-svn-id: http://core.svn.wordpress.org/trunk@37712 1a063a9b-81f0-0310-95a4-ce76da25c4cd