[PLT-6838] Restrict channel delete option per permission policy even for last channel member (#6706)

* channel delete option is hidden from the menu unless there is appropriate permissions as set in the policy page

* apply to public channel only and add restriction to API layer

* updated channel deletion

webapp/components/channel_header.jsx

@@ -720,11 +720,7 @@ export default class ChannelHeader extends React.Component {
                 );
             }
 
-            if (ChannelUtils.showDeleteOption(channel, isAdmin, isSystemAdmin, isChannelAdmin)) {
-                if (!ChannelStore.isDefault(channel)) {
-                    dropdownContents.push(deleteOption);
-                }
-            } else if (this.state.userCount === 1) {
+            if (ChannelUtils.showDeleteOption(channel, isAdmin, isSystemAdmin, isChannelAdmin, this.state.userCount)) {
                 dropdownContents.push(deleteOption);
             }
 

webapp/components/delete_channel_modal.jsx

@@ -33,6 +33,7 @@ export default class DeleteChannelModal extends React.Component {
 
         browserHistory.push(TeamStore.getCurrentTeamRelativeUrl() + '/channels/town-square');
         deleteChannel(this.props.channel.id);
+        this.onHide();
     }
 
     onHide() {

webapp/components/navbar.jsx

@@ -529,23 +529,21 @@ export default class Navbar extends React.Component {
                     );
                 }
 
-                if (ChannelUtils.showDeleteOption(channel, isAdmin, isSystemAdmin, isChannelAdmin) || this.state.userCount === 1) {
-                    if (!ChannelStore.isDefault(channel)) {
-                        deleteChannelOption = (
-                            <li role='presentation'>
-                                <ToggleModalButton
-                                    role='menuitem'
-                                    dialogType={DeleteChannelModal}
-                                    dialogProps={{channel}}
-                                >
-                                    <FormattedMessage
-                                        id='channel_header.delete'
-                                        defaultMessage='Delete Channel'
-                                    />
-                                </ToggleModalButton>
-                            </li>
-                        );
-                    }
+                if (ChannelUtils.showDeleteOption(channel, isAdmin, isSystemAdmin, isChannelAdmin, this.state.userCount)) {
+                    deleteChannelOption = (
+                        <li role='presentation'>
+                            <ToggleModalButton
+                                role='menuitem'
+                                dialogType={DeleteChannelModal}
+                                dialogProps={{channel}}
+                            >
+                                <FormattedMessage
+                                    id='channel_header.delete'
+                                    defaultMessage='Delete Channel'
+                                />
+                            </ToggleModalButton>
+                        </li>
+                    );
                 }
 
                 const canLeave = channel.type === Constants.PRIVATE_CHANNEL ? this.state.userCount > 1 : true;