Discord: Add option to provide server invite

When enabled, a temporary one-use invite is created and shown to the
user on the account creation form.

html/form-base.html

@@ -20,6 +20,8 @@
     window.discordEnabled = {{ .discordEnabled }};
     window.discordRequired = {{ .discordRequired }};
     window.discordPIN = "{{ .discordPIN }}";
+    window.discordInviteLink = {{ .discordInviteLink }};
+    window.discordServerName = "{{ .discordServerName }}";
 </script>
 <script src="js/form.js" type="module"></script>
 {{ end }}