PLT-6484 Add /leave command to leave a channel (#6402)

* PLT-6484 Add /leave command to leave a channel

* Text changes requeted on review.

* PLT-6484 Display the right error message when trying to /leave town-square

* PLT-6484 Be able to execute /leave command in direct and group message channels with the same effect as clicking x

* PLT-6484 Refactor to create new leave_private_channel_modal.jsx

* PLT-6484 Remove previous leave private channel logic to use new leave_private_channel_modal.jsx

* Remove dot in command description. Change localized error when leaving Town square.

* disable /leave command in reply threads on the right-hand sidebar, since it is not obvious which channel you should leave

webapp/stores/modal_store.jsx

@@ -44,6 +44,7 @@ class ModalStoreClass extends EventEmitter {
         case ActionTypes.TOGGLE_CHANNEL_HEADER_UPDATE_MODAL:
         case ActionTypes.TOGGLE_CHANNEL_PURPOSE_UPDATE_MODAL:
         case ActionTypes.TOGGLE_CHANNEL_NAME_UPDATE_MODAL:
+        case ActionTypes.TOGGLE_LEAVE_PRIVATE_CHANNEL_MODAL:
             this.emit(type, value, args);
             break;
         }