[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/utils/channel_utils.jsx

@@ -190,11 +190,15 @@ export function showManagementOptions(channel, isAdmin, isSystemAdmin, isChannel
     return true;
 }
 
-export function showDeleteOption(channel, isAdmin, isSystemAdmin, isChannelAdmin) {
+export function showDeleteOption(channel, isAdmin, isSystemAdmin, isChannelAdmin, userCount) {
     if (global.window.mm_license.IsLicensed !== 'true') {
         return true;
     }
 
+    if (ChannelStore.isDefault(channel)) {
+        return false;
+    }
+
     if (channel.type === Constants.OPEN_CHANNEL) {
         if (global.window.mm_config.RestrictPublicChannelDeletion === Constants.PERMISSIONS_SYSTEM_ADMIN && !isSystemAdmin) {
             return false;
@@ -206,6 +210,9 @@ export function showDeleteOption(channel, isAdmin, isSystemAdmin, isChannelAdmin
             return false;
         }
     } else if (channel.type === Constants.PRIVATE_CHANNEL) {
+        if (userCount === 1) {
+            return true;
+        }
         if (global.window.mm_config.RestrictPrivateChannelDeletion === Constants.PERMISSIONS_SYSTEM_ADMIN && !isSystemAdmin) {
             return false;
         }