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/components/needs_team/needs_team.jsx

@@ -45,6 +45,7 @@ import ImportThemeModal from 'components/user_settings/import_theme_modal.jsx';
 import InviteMemberModal from 'components/invite_member_modal.jsx';
 import LeaveTeamModal from 'components/leave_team_modal.jsx';
 import ResetStatusModal from 'components/reset_status_modal';
+import LeavePrivateChannelModal from 'components/modals/leave_private_channel_modal.jsx';
 
 import iNoBounce from 'inobounce';
 import * as UserAgent from 'utils/user_agent.jsx';
@@ -231,6 +232,7 @@ export default class NeedsTeam extends React.Component {
                     <DeletePostModal/>
                     <RemovedFromChannelModal/>
                     <ResetStatusModal/>
+                    <LeavePrivateChannelModal/>
                 </div>
             </div>
         );