discord: modularize user-facing code

will be done for others too, code for discord account linking in form
and userpage is now in ts/modules/account-linking.ts as a configurable
class.

ts/modules/common.ts

@@ -221,8 +221,3 @@ export function insertText(textarea: HTMLTextAreaElement, text: string) {
         textarea.focus();
     }
 }
-
-export interface DiscordInvite {
-    invite: string;
-    icon: string;
-}