Bug 1024527 - Move search_strings.dtd into m/a/base/locales/en-US.
Bug 1024527 - Post: Simplify |grunt export|.

As much as possible, we want app/src/main to be exactly what gets
shipped in mozilla-central.  To that end, this patch moves things that
do not get exported to mozilla-central into app/src/external.  This both
simplifies |grunt export| but also future-proofs the logic.
Bug 1024527 - Move search_strings.dtd into m/a/base/locales/en-US. r=margaret