prevent inserting truncated UTF-8 into motto field

[Jamiras]

Reccurrance of:

Error encoding model [App\\Site\\Models\\User] with ID  to JSON: Malformed UTF-8 characters, possibly incorrectly encoded 
at /home/forge/retroachievements.org/releases/2023-08-14T082503-3.2.2-83eca4ee/vendor/laravel/framework/src/Illuminate/Database/Eloquent/JsonEncodingException.php:18)

Which was originally reported here.

This sanitizes the motto going into the database so the error shouldn't occur again in the future.

[wescopeland]

Verified locally the the mb_strcut() call is working correctly. I did not verify the other change, but I understand what it's doing and agree it's desirable.