`Thread::addParticipant` ignores soft-deleted participants

[LucasBrunner]

Thread::addParticipant does not check to see if a matching soft-deleted participants record already exists and will create a duplicate record. Thread::activateAllParticipants can then restore the deleted record.

When multiple active participants records exist for a single user-thread pair the example project displays unexpected behavior, such as showing threads multiple times.

This could be fixed by updating Thread::addParticipant to check for existing soft-deleted records.

Example code:

use App\Models\User;
use Cmgmyr\Messenger\Models\Participant;
use Cmgmyr\Messenger\Models\Thread;

$user = User::create([
    'name' => 'myself',
    'email' => 'my@self.com',
    'password' => bcrypt('password'),
]);

$thread = Thread::create(['subject' => 'Duplicate Participant']);
echo $thread->participants()->count(); // 0

$thread->addParticipant($user->id);
echo $thread->participants()->count(); // 1

$thread->participants()->delete();
echo $thread->participants()->count(); // 0

$thread->addParticipant($user->id);
echo $thread->participants()->count(); // 1

$thread->activateAllParticipants();
echo $thread->participants()->count(); // 2