userpage: use form langfile, move login strings to common

login-related stuff was moved into common using the langmover script, so
that the user page doesn't have to use the admin language files.

lang/common/en-us.json

@@ -24,6 +24,14 @@
         "contactDiscord": "Contact through Discord",
         "theme": "Theme",
         "refresh": "Refresh",
-        "required": "Required"
+        "required": "Required",
+        "login": "Login",
+        "logout": "Logout"
+    },
+    "notifications": {
+        "errorLoginBlank": "The username and/or password were left blank.",
+        "errorConnection": "Couldn't connect to jfa-go.",
+        "errorUnknown": "Unknown error.",
+        "error401Unauthorized": "Unauthorized. Try refreshing the page."
     }
-}
+}