[5.6] Register many observers for a model

[Alymosul]

This PR adds the ability to pass an array of observers to the observe() method for a model instead of registering each of them one by one.
Consider we have a Thread model and we will register the following observers for it: DeleteThreadRepliesObserver, GenerateThreadSlugObserver, ReputationObserver, ThreadObserver

Currently:
\App\Thread::observe(\App\Observers\DeleteThreadRepliesObserver::class);
\App\Thread::observe(\App\Observers\GenerateThreadSlugObserver::class);
\App\Thread::observe(\App\Observers\ReputationObserver::class);
\App\Thread::observe(\App\Observers\ThreadObserver::class);

Proposed:
\App\Thread::observe([
\App\Observers\DeleteThreadRepliesObserver::class,
\App\Observers\GenerateThreadSlugObserver::class,
\App\Observers\ReputationObserver::class,
\App\Observers\ThreadObserver::class
]);

[ntzm]

This is nice, I've definitely needed this before! However, could you possibly add tests? Thanks

[devcircus]

I second this. Though I personally no longer use observers in most cases. It makes sense. Tests would be nice.

[ludo237]

@devcircus is there a better alternative to observers?

@Alymosul I think you should add tests in order to verify that someone can actually use the old version and the new one

[afraca]

Nice addition.
Nitpick: I know this is already merged, but perhaps we could make the registerObserver method protected to be in line with the rest of the model and be easier to override?