(Bug 33181) Special:Concepts to list available concepts

I got relentlessly tiered of searching for existing concepts therefore
this Special:Concepts should solve the issue.

* Fix "Use of Linker::makeLinkObj was deprecated in MediaWiki 1.21" in
SMW_DV_WikiPage.php:287 due to the complained of PHPunit.

Test coverage: 100%
CRAP: 6

Change-Id: I4901a2814e9e9c514dbd2b5abf4be74002319f6e