Reenable regexes now that we've fixed the plugin

Before late last night San Francisco time it was possible to send regexes
that could consume all the memory on an Elasticsearch node.  Now we've
finished rolling out a plugin update that makes that impossible.

A quick note on order of operations: to make sure there isn't any time
where regexes can still overwhelm the system sync CirrusSearch-common.php
first.

Bug 72894

Change-Id: Id01271785d3d2e0554296b6df4acfb56c139a810
Merge "Reenable regexes now that we've fixed the plugin"