* 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
