Introduces monitoring of multiple files for cache invalidation

* adds composer.lock to default list of files monitored for cache triggering
* adds 'cachetriggers' key to keys recognized by ResourceLoaderBootstrapModule
* adds 'cachetriggers' key to $wgResourcemodules[ 'ext.bootstrap.styles' ]

Change-Id: I32a198f1b6f34d8fa93c4e650cd2f3f6f781c5de