The `ember_jquery` bundle contains production builds of Ember and jQuery
which doesn't work with tests. This commits introduces a new
`theme_qunit_vendor` bundle which is copy of the `vendor` bundle but
doesn't contain `ember_jquery`.
This commit is a partial revert of
https://github.com/discourse/discourse/commit/409c8585e46250ee495356e1897a4fda905152b3
In Ember CLI, the vendor bundler includes Ember/jQuery, so this brings
our app closer to that configuration.
We have a couple pages (Reset Password / Confirm New Email) where we need
`ember_jquery` without vendor so the file still exists for those cases.