Tag pager query to allow hook sorting of submissions

Problem/Motivation

Using a custom module, we hide the username and use firstname/lastname etc. fields instead. These display as they should when viewing webform submissions, but of course clicking to sort the column uses the underlying value.

Proposed resolution

Tagging the $pager_query in webform.submissions.inc would allow a hook to alter the query. I'll submit the patch I did locally once I have the issue number.

Even though it's so trivial, it's my first, so let me know of anything I should improve. Thank you!