Fix conference search error

[andrewmile]

This PR fixes an error that occurring when the Laravel\Scout\Jobs\MakeSearchable job is queued for a conference. These jobs are failing with a MaxAttemptsExceededException being thrown. These jobs are inserted in the failed_jobs table with the correct conference ID but the underlying query is recorded in Bugsnag as follows:

select * from `conferences` where `conferences`.`id` in (0)

This Scout issue led me to the Laravel\Scout\Searchable::queryScoutModelsByIds method, which determine's the model's key via the Model::getKeyType method. When the key type is int, which is the Eloquent default, the query is built with whereIntegerInRaw, which cast's the key as an int.

Since the Conference model uses a UUID primary key, the queryScoutModelsByIds method was casting the UUID value as an int, which resulted in the SQL query above.

Adding protected $keyType = 'string'; to the App\Models\UuidBase class results in the queryScoutModelsByIds method using a whereIn query instead of the whereIntegerInRaw, which fixes this issue.

This fix is tested by using collection as the SCOUT_DRIVER for PHPUnit since the database driver doesn't call the queryScoutModelsByIds method.