PLT-3156 Typo in 'Edit Nickname' message when nickname is set through login provider (#3204)

* Fixed a typo to an error message regarding editing an account field handled by a login provider. (PLT-3156)

* Added 'field_handled_externally' to the en language file.
Other languages are needed.

webapp/components/user_settings/user_settings_general.jsx

@@ -596,7 +596,7 @@ class UserSettingsGeneralTab extends React.Component {
                     <span>
                         <FormattedMessage
                             id='user.settings.general.field_handled_externally'
-                            defaultMessage='This field is handled through your login provider. If you want to change it, you need to do so though your login provider.'
+                            defaultMessage='This field is handled through your login provider. If you want to change it, you need to do so through your login provider.'
                         />
                     </span>
                 );