PLT-7206: Remove the "Delete Channel" option for private channels if you're the last channel member and policy setting restricts channel deletion (#7314)

* Removed restriction that prevented last occupant of a channel from leaving. Any user can now leave any channel, except for the default Town Square channel

* Also removed last user leave limitation from navbar

webapp/components/channel_header.jsx

@@ -680,8 +680,7 @@ export default class ChannelHeader extends React.Component {
                 );
             }
 
-            const canLeave = channel.type === Constants.PRIVATE_CHANNEL ? this.state.userCount > 1 : true;
-            if (!ChannelStore.isDefault(channel) && canLeave) {
+            if (!ChannelStore.isDefault(channel)) {
                 dropdownContents.push(
                     <li
                         key='divider-3'

webapp/components/navbar.jsx

@@ -555,8 +555,7 @@ export default class Navbar extends React.Component {
                     );
                 }
 
-                const canLeave = channel.type === Constants.PRIVATE_CHANNEL ? this.state.userCount > 1 : true;
-                if (!ChannelStore.isDefault(channel) && canLeave) {
+                if (!ChannelStore.isDefault(channel)) {
                     leaveChannelOption = (
                         <li role='presentation'>
                             <a