Modal open calls should resolve in order

[nlwillia]

Modal instances should be added to the stack in the same order as their $modal.open calls. Currently, due to template and resolve resolution, modal instances are added in the order that the promises they depend on are fulfilled. This can be a problem if modals are linked to router states and a series of nested modals each with its own deferred dependencies is launched by a deep state link. The resulting display layer order should be deterministic, but instead it is dependent on which promises resolve first.

Here is a demo of the problem in action. The modals are opened in order from 1-3, but the resolve order is 3-1 and modal 1 ends up on top of the stack instead of the bottom.

When opening a new modal, its promises should be chained off the promise of the previous modal opened so that no modal is added to the stack until all of its predecessors have successfully resolved.

[nlwillia]

I've submitted #2443 with a test case and fix.