[PLT-6707] /header [text] slash command: Edit the channel header (#6566)

* add /header slash command

* update websocket

* updater per review

webapp/actions/global_actions.jsx

@@ -212,6 +212,14 @@ export function showDeletePostModal(post, commentCount = 0) {
     });
 }
 
+export function showChannelHeaderUpdateModal(channel) {
+    AppDispatcher.handleViewAction({
+        type: ActionTypes.TOGGLE_CHANNEL_HEADER_UPDATE_MODAL,
+        value: true,
+        channel
+    });
+}
+
 export function showGetPostLinkModal(post) {
     AppDispatcher.handleViewAction({
         type: ActionTypes.TOGGLE_GET_POST_LINK_MODAL,

webapp/actions/websocket_actions.jsx

@@ -229,6 +229,11 @@ function handleEvent(msg) {
     }
 }
 
+function handleChannelUpdatedEvent(msg) {
+    const channel = JSON.parse(msg.data.channel);
+    dispatch({type: ChannelTypes.RECEIVED_CHANNEL, data: channel});
+}
+
 function handleNewPostEvent(msg) {
     const post = JSON.parse(msg.data.post);
     handleNewPost(post, msg);
@@ -341,11 +346,6 @@ function handleUserRemovedEvent(msg) {
     }
 }
 
-function handleChannelUpdatedEvent(msg) {
-    const channel = JSON.parse(msg.data.channel);
-    dispatch({type: ChannelTypes.RECEIVED_CHANNEL, data: channel});
-}
-
 function handleUserUpdatedEvent(msg) {
     const user = msg.data.user;
     if (UserStore.getCurrentId() !== user.id) {