form: reliably disable submit button, communicate if account linking is required

lang/common/en-us.json

@@ -23,6 +23,7 @@
         "linkMatrix": "Link Matrix",
         "contactDiscord": "Contact through Discord",
         "theme": "Theme",
-        "refresh": "Refresh"
+        "refresh": "Refresh",
+        "required": "Required"
     }
 }

lang/form/en-us.json

@@ -31,6 +31,8 @@
         "errorUnknown": "Unknown error.",
         "errorNoEmail": "Email required.",
         "errorCaptcha": "Captcha incorrect.",
+        "errorPassword": "Check password requirements.",
+        "errorNoMatch": "Passwords don't match.",
         "verified": "Account verified."
     },
     "validationStrings": {