add 'mwgrep' common script

mwgrep takes a search term string as an argument. It then searches through the
MediaWiki namespace of all Wikimedia wikis for that term, using the
ElasticSearch cluster that powers CirrusSearch.

Change-Id: Ie8bf4cdc3c91c4d94891f5fab174350f300da9fe
Merge "add 'mwgrep' common script" into production