invites: add "User Label" 1/2

Adds a "User Label" invite field, which is a label applied to users
created with it. This commit contains everything apart from the code to
apply it on account creation.

ts/typings/d.ts

@@ -129,6 +129,7 @@ interface Invite {
     notifyCreation?: boolean;
     profile?: string;
     label?: string;
+    user_label?: string;
     userExpiry?: boolean;
     userExpiryTime?: string;
 }