[PLT-6708] /purpose [text] slash command: Edit the channel purpose (#6569)

* /purpose [text] slash command: Edit the channel purpose

* update command on server side to check for direct or group channels

* update stings and block the dialog when is DM or GM

* update per review

* remove duplicate websocker event and apply the same for /header command

* update per review

* update

webapp/utils/constants.jsx

@@ -175,6 +175,7 @@ export const ActionTypes = keyMirror({
     TOGGLE_DM_MODAL: null,
     TOGGLE_QUICK_SWITCH_MODAL: null,
     TOGGLE_CHANNEL_HEADER_UPDATE_MODAL: null,
+    TOGGLE_CHANNEL_PURPOSE_UPDATE_MODAL: null,
 
     SUGGESTION_PRETEXT_CHANGED: null,
     SUGGESTION_RECEIVED_SUGGESTIONS: null,