[8.x] Add support for disallowing class morphs

[olivernybroe]

This PR adds a new prevention flag to models. This flag is for disallowing morphs without a morph map on it.

For enabling it simply do a

Model::preventClassMorphs();

When a model does not have a morh map registered a ClassMorphViolationException will be thrown.

For preventing the exception, simply register the model in the morph map

Relation::morphMap([
    'user' => User::class,
]);

[driesvints]

Were the concerns from #38451 taken into account on this PR?

[olivernybroe]

@driesvints Did not know about that.
But as far as I understand from the PR, the issue is when you are getting the data from the database, it needs a map to lookup what model the string in the database corresponds to.

The feature I am proposing here should not be impacted for that, as this flag simply prevents you from saving a morph relation without a morphmap, it does not prevent you from loading a model without a morphmap.

So you can still load a model from the database where the column value is a FQCN, it is only the saving part that is prevented.

[aarondfrancis]

@olivernybroe this is wonderful! I've been enforcing this myself via a trait aaronfrancis.com/2020/laravel-consistent-morphs but obviously would much rather this be first party.

Here is a thread where a couple of folks talk about their need for this feature: https://twitter.com/aarondfrancis/status/1427399215479672837.

The issue this would solve is that while it is simple to understand that if you change a class name you need to update everything in the DB, it's also extremely easy to forget. And when you have a big team with a huge application, that problem becomes much worse.

This makes the implicit explicit, which is super helpful for teams!

Thanks Oliver!

[taylorotwell]

Moved the method to Relation like morphMap.

Added new method enforceMorphMap that can be used to define a morph map and turn off class mapping in one method call instead of two:

Relation::enforceMorphMap([
    'user' => User::class,
]);