Fix emoji insertion on button label

Author: NMertsch

src/europython_discord/registration/cog.py

@@ -15,7 +15,8 @@
 
 _logger = logging.getLogger(__name__)
 
-REGISTRATION_BUTTON_LABEL = "Register here :point_left:"
+# Discord's colon-syntax `:point_left: does not work in button labels`, so we use Python's syntax
+REGISTRATION_BUTTON_LABEL = "Register here \N{WHITE LEFT POINTING BACKHAND INDEX}"
 WELCOME_MESSAGE_TITLE = "## Welcome to EuroPython 2025 on Discord! :tada::snake:"