Next release : better emoji support out of the box

Some foundations are right, like storing everything as utf8mb4 in the DB. Yet, I have the feeling we could be more liberal with the emojis and funky chars, with the following goals :

• out of the box, accept any custom keyword (ozh, O-Z-H, ozh♥️, etc, with most likely some chars still reserved such as ;#{} and a few more)
• this should work flawlessly on stat pages, editing keyword, etc
• continue to generate random keywords against an ASCII set [a-z0-9] (uppercase?)
• eventually maybe deprecated YOURLS_URL_CONVERT because frankly picking 36 or 62 hasn't be very user friendly