Followup to r91608: reduce impact of bug 29784 (high jsmin+ memory usage during parsi...

Followup to r91608: reduce impact of bug 29784 (high jsmin+ memory usage during parsing) by skipping JS validation on modules backed by files. Unless you're developing them, these will usually be stable, and large individual files like the pre-bundled jQuery can hit memory limits much sooner than we like.

Validation on JS from wiki pages is still on by default ($wgResourceLoaderValidateJS = true).
Validation on static files can be re-enabled by setting $wgResourceLoaderValidateStaticJS = true (defaults false).