PHPCS sniff for un-restored switch_to_blog() calls
switch_to_blog() is called but not properly "closed" via
restore_current_blog(), havoc ensues. The most recent instance of this is #15170.
We should introduce a PHPCS sniff to our custom ruleset that looks for
switch_to_blog(), and then ensures that all code paths out of the method have a
restore_current_blog() before returning.