Attendance UI search does not handle apostrophes

[johneckman]

If your attendee name is O'Brien or O'Donnell - you won't be able to find them in the Attendance UI

If you just search for Brien or Donnell it will find them, but if you try to type O'Brien it will not.

[johneckman]

showing the o' attendees

[johneckman]

Showing the filter not finding them

[jeffSLOfish]

Fix for this bug.

[BandonRandon]

Tested Patch, works as expected. cc @kovshenin

[coreymckrill]

In 5531:

WordCamp: Handle apostrophes in Camptix Attendance UI

Apostrophes in the search string are getting escaped when they are first
submitted, which means they are getting escaped again when they go
through $wpdb->prepare etc. So strings that have an apostrophe (like
"O'Brien") will not turn up the expected results in the query.

This fixes the issue by stripping slashes before the string goes through
esc_like and $wpdb->prepare. It uses wp_unslash instead of the
stripslashes in the original patch simply because using WP-specific
versions of functions is a best practice.

Stripping slashes in this instance doesn't cause a vulnerability because
the escaping is handled again later by esc_like and $wpdb->prepare.

Props jeffSLOfish
Fixes #1856