Make the ticket purchase confirmation email chat server agnostic (#1354)

* Made the confirmation email char server-agnostic

Author: fpierfed

templates/conference/emails/order_confirmation_email.txt

@@ -20,16 +20,11 @@ steps to have them assigned to your attendees:
 
 https://ep2021.europython.eu/faq/#I-bought-the-ticket-for-someone-else-How-can-I-re-assign-it
 
-We will be running the conference using our Discord server. In order
-to register, please use the Discord invite link listed on our
-website:
+We will be running the conference using our chat server. Please follow
+the instructions on our website:
 
 https://ep2021.europython.eu/setup/chat-system/
 
-and follow the instructions on the page to register. We also provide
-a helpful intro video to get you started with Discord on the same
-page.
-
 If you run into any issues, please contact us at [email protected]
 
 Kind regards,