feat: eloquent factories (primarily for tests)

Please ignore the failing PHPStan and Frontend tests, those have been fixed in other PRs that are still open

Changes proposed in this pull request: This PR introduces some eloquent factories for some models. The primary reason for introducing this is the following use case within tests:

We have a very large number of tests, in most tests we declare the data we need in the database like so:

$this->prepareDatabase([
    'discussions' => [
        ['id' => 1, 'title' => 'no tags', 'user_id' => 1, 'comment_count' => 1],
        ['id' => 2, 'title' => 'has tags where mods can view discussions but not flags', 'user_id' => 1, 'comment_count' => 1],
        ['id' => 3, 'title' => 'has tags where mods can view flags but not discussions', 'user_id' => 1, 'comment_count' => 1],
        ['id' => 4, 'title' => 'has tags where mods can view discussions and flags', 'user_id' => 1, 'comment_count' => 1],
        ['id' => 5, 'title' => 'has unrestricted tag', 'user_id' => 1, 'comment_count' => 1],
    ],
]);

We set `comment_count' to 1 almost always rather than not, so that we are certain the discussions are visible. The problem is that when you introduce a new column that requires you to similarly always give it a value, you have to go through all the data in all the tests and make changes accordingly.. Which is very inconvenient.

By introducing factories, each model has a default template of fake data that can be edited, and the data specified in the tests is passed to the model factory to generate the final data.

So with this PR, it becomes as follows:

$this->prepareDatabase([
    Discussion::class => [
        ['id' => 1, 'title' => 'no tags', 'user_id' => 1],
        ['id' => 2, 'title' => 'has tags where mods can view discussions but not flags', 'user_id' => 1],
        ['id' => 3, 'title' => 'has tags where mods can view flags but not discussions', 'user_id' => 1],
        ['id' => 4, 'title' => 'has tags where mods can view discussions and flags', 'user_id' => 1],
        ['id' => 5, 'title' => 'has unrestricted tag', 'user_id' => 1],
    ],
]);

// each row will be passed to the model factory like so internally:
$row = $modelClass::factory()->raw($row);

// The factory will have the default fake value needed.
class DiscussionFactory extends Factory
{
    public function definition(): array
    {
        return [
            'title' => $this->faker->sentence,
            'comment_count' => 1,
            ...
        ];
    }
}

To implement a factory class, the model must use the HasFactory trait, and the factory class which extends Illuminate\Database\Eloquent\Factories\Factory can exist in the same namespace as the model class. Otherwise the model may specify the class of the factory through the static newFactory method.

More on factories here: https://laravel.com/docs/10.x/eloquent-factories.

Additionally, factories are useful for other features the ecosystem may build (an example can involve a previewing feature of sorts).

Reviewers should focus on:

Screenshot

Necessity

[ ] Has the problem that is being solved here been clearly explained?
[ ] If applicable, have various options for solving this problem been considered?
[ ] For core PRs, does this need to be in core, or could it be in an extension?
[ ] Are we willing to maintain this for years / potentially forever?

Confirmed

[ ] Frontend changes: tested on a local Flarum installation.
[ ] Backend changes: tests are green (run composer test).
[ ] Core developer confirmed locally this works as intended.
[ ] Tests have been added, or are not appropriate here.

Required changes:

[ ] Related documentation PR: (Remove if irrelevant)
[ ] Related core extension PRs: (Remove if irrelevant)

flarum / framework

feat: eloquent factories (primarily for tests) #3982