PLT-5873 Renamed private groups to private channels (#5924)

* PLT-5873 Renamed private groups to private channels in web app

* PLT-5873 Renamed private groups to private channels in server

webapp/utils/channel_intro_messages.jsx

@@ -217,13 +217,13 @@ export function createStandardIntroMessage(channel, centeredIntro) {
         uiType = (
             <FormattedMessage
                 id='intro_messages.group'
-                defaultMessage='private group'
+                defaultMessage='private channel'
             />
         );
         memberMessage = (
             <FormattedMessage
                 id='intro_messages.onlyInvited'
-                defaultMessage=' Only invited members can see this private group.'
+                defaultMessage=' Only invited members can see this private channel.'
             />
         );
     } else {