Create endpoint to dump cirrus configuration

Convenient for debugging and useful to anyone who might want to look at
how we've configured it on a specific wiki.  The api can be shut off by
setting $wgCirrusSearchEnableConfigDumpApi=false;

Also removed an unused config parameter and renamed a confusing one.

Bug: 69442
Change-Id: I744690ee1c1ffb31f85fb96f641ba958b77a1629
Merge "Create endpoint to dump cirrus configuration"