feat(alert): add config service to provide default values

[jnizet]

refs #518

I suffered with the tests of NgbDismissibleAlert because creating an instance of this component using new or with the TestBed is harder than with other components: it has many dependencies thatare used in the constructor. Instead of trying to mock them all or to change the code of the component just to ease that test, I thus used a slightly different test strategy than for the other components.

[wesleycho]

This seems weird to me - does it need to be injected here? Can you explain a little more in detail about what situation this is simulating?

[jnizet]

The idea is to simulate what the config service documentation suggests to do to globally change the default values for alerts (and all the other components):

• NgbAlertModule declares an NgbAlertConfig provider
• an application main module (simulated by the test module here) imports NgbAlertModule (or NgbModule). A global NgbAlertConfig service, containing mutable default values, is thus available to the whole application
• the root application component gets a reference to this global NgbAlertConfig config thanks to dependency injection. That is simulated by the inject() in the test
• the root application component sets custom default values in the injected NgbAlertConfig service. That is simulated by the body of the function passed to inject().
• A template of the application ends up using an NgbAlert component. This is simulated by the createTestComponent() call in the test
• The created alert should use the default values set before in the NgbAlertConfig service. This is what the test checks.

Does that clarify? I agree the test is more convoluted than the other similar tests, but checking for the right timeout can't be done by just inspecting the dom, and, to be fair, I haven't found a way to just getting an instance of the NgbDismissibleAlert in the test.

Using

TestBed.createComponent(NgbDismissibleAlert)

leads to the following error message, which looks completely wrong to me:

Cannot create the component NgbDismissibleAlert as it was not imported into the testing module!

[wesleycho]

This LGTM, just want a little more clarification on the test scenario.

[icfantv]

@wesleycho are you satisfied with the explanation?
@jnizet please rebase and fix the conflicts. thanks.

[wesleycho]

Thanks for the very detailed explanation - I'm ok with this then.

[jnizet]

Rebase done. I also added the missing api doc for the two config service un the demo and fixed the label of the config demo. :-/