userpage: add/edit discord

works identically to on the form, would like to eventually factor out
the discord/telegram/matrix verif stuff so it can be shared between the
two pages though.

ts/modules/common.ts

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