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/utils/constants.jsx

@@ -180,6 +180,7 @@ export const ActionTypes = keyMirror({
     TOGGLE_CHANNEL_HEADER_UPDATE_MODAL: null,
     TOGGLE_CHANNEL_PURPOSE_UPDATE_MODAL: null,
     TOGGLE_CHANNEL_NAME_UPDATE_MODAL: null,
+    TOGGLE_LEAVE_PRIVATE_CHANNEL_MODAL: null,
 
     SUGGESTION_PRETEXT_CHANGED: null,
     SUGGESTION_RECEIVED_SUGGESTIONS: null,