[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

i18n/en.json

@@ -571,6 +571,38 @@
     "id": "api.command_echo.name",
     "translation": "echo"
   },
+  {
+    "id": "api.command_channel_purpose.name",
+    "translation": "purpose"
+  },
+  {
+    "id": "api.command_channel_purpose.hint",
+    "translation": "[text]"
+  },
+  {
+    "id": "api.command_channel_purpose.desc",
+    "translation": "Edit the channel purpose"
+  },
+  {
+    "id": "api.command_channel_purpose.channel.app_error",
+    "translation": "Error to retrieve the current channel."
+  },
+  {
+    "id": "api.command_channel_purpose.permission.app_error",
+    "translation": "You do not have the appropriate permissions to edit the channel purpose."
+  },
+  {
+    "id": "api.command_channel_purpose.direct_group.app_error",
+    "translation": "Cannot set purpose for direct message channels. Use /header to set the header instead."
+  },
+  {
+    "id": "api.command_channel_purpose.message.app_error",
+    "translation": "A message must be provided with the /purpose command."
+  },
+  {
+    "id": "api.command_channel_purpose.update_channel.app_error",
+    "translation": "Error to update the current channel."
+  },
   {
     "id": "api.command_expand.desc",
     "translation": "Turn off auto-collapsing of image previews"