feat(modal): Adds defaultContainer to $modalProvider

[Lalem001]

Includes basic tests, with adjustments to toHaveModalsOpen and toHaveBackdrop.

Fixes: #2688
Replaces PR: #2942

[wesleycho]

Looking at the original issue this PR references, any particular reason for the choice of container over appendTo? I think the latter makes more sense semantically, but am open to other options.

[Lalem001]

container is just coming off of my use of defaultContainer. Would you want appendTo and defaultContainer? Or change both to appendTo and defaultAppendTo?

Or perhaps something else entirely?

[wesleycho]

It should be appendTo and there should be no defaultContainer reference.

[wesleycho]

Should be noted, we now have an appendTo feature via 16d854c - this feature should interop with the new implementation. Given that it takes a DOM node in the $modal service, I think this feature then becomes invalid. If one needs a sensible default, I recommend creating a modal abstraction on top of $modal to pass the desired defaults.