Make the minimum user password length configurable

Previously, all passwords had to be at least 12 characters long.

Some users use a slightly shorter password, so some instances could wish
to reduce the password length limit to a slightly lesser value.

This makes the minimum password length configurable in the .env file,
with the MIN_PASSWORD_LENGTH variable, that defaults to 12 to keep the
previous behaviour by default.

app/Http/Controllers/Auth/RegisterController.php

@@ -115,7 +115,7 @@ class RegisterController extends Controller
             'name'     => 'nullable|string|max:'.config('pixelfed.max_name_length'),
             'username' => $usernameRules,
             'email'    => $emailRules,
-            'password' => 'required|string|min:12|confirmed',
+            'password' => 'required|string|min:'.config('pixelfed.min_password_length').'|confirmed',
         ];
 
         return Validator::make($data, $rules);