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.

lang/admin/en-us.json

@@ -41,6 +41,8 @@
         "profile": "Profile",
         "unknown": "Unknown",
         "label": "Label",
+        "userLabel": "User Label",
+        "userLabelDescription": "Label to apply to users created with this invite.",
         "logs": "Logs",
         "announce": "Announce",
         "templates": "Templates",
@@ -223,4 +225,4 @@
             "plural": "Extended expiry for {n} users."
         }
     }
-}
+}