This commit add $.create(), which allows you to create a
jQuery object that just has a name to identify it, as opposed
to some selector or HTML fragment. It's useful for things that
are really used as stubs.
This also fixes a bunch of the existing tests to use $.create().
Before this fix, you could actually just do $('some-stub'), but
now we enforce that the input to $() looks like a valid selector
or HTML fragment, and we make some exceptions for things like
window-stub and document-stub.
This test verifies the following line of code:
$(document).trigger($.Event('subscription_remove_done.zulip', {sub: sub}));
Before this change, the mocking mechanism used `$(document).on(...)`
to set the function that gets called on trigger, but it didn't clear
any of the other handlers.
Since all we care about is making sure that the event gets triggered,
we now just override `$(document).trigger`.
