PLT-7206: Remove the "Delete Channel" option for private channels if you're the last channel member and policy setting restricts channel deletion (#7050)

* PLT-7206: UI changes. Removed last user in channel loophole, refactored code to clean it up, added differentiated support for public and private channels, added unit tests. Still need to implement server-side checks

* PLT-7206: All helper methods in channel_utils.jsx now accept the same three boolean variables in the same order and use the same boolean logic to check their values.

* PLT-7206: Added unit tests for showManagementOptions(...)

* PLT-7206: Fixed test case descriptions

* Added unit tests for showCreateOption(...)

* PLT-7206: Added unit tests for canManageMembers(...)

* PLT-7206: Removed last person in channel loophole from server-side code

* PLT-7206: Reverted config.json

* PLT-7206: Fixed double negatives in unit test names

* PLT-7206: PR feedback - Removed confusing comment and unused variable

webapp/components/sidebar.jsx

@@ -714,7 +714,7 @@ export default class Sidebar extends React.Component {
             />
         );
 
-        const isAdmin = TeamStore.isTeamAdminForCurrentTeam() || UserStore.isSystemAdminForCurrentUser();
+        const isTeamAdmin = TeamStore.isTeamAdminForCurrentTeam();
         const isSystemAdmin = UserStore.isSystemAdminForCurrentUser();
 
         let createPublicChannelIcon = (
@@ -753,11 +753,11 @@ export default class Sidebar extends React.Component {
             </OverlayTrigger>
         );
 
-        if (!ChannelUtils.showCreateOption(Constants.OPEN_CHANNEL, isAdmin, isSystemAdmin)) {
+        if (!ChannelUtils.showCreateOption(Constants.OPEN_CHANNEL, isTeamAdmin, isSystemAdmin)) {
             createPublicChannelIcon = null;
         }
 
-        if (!ChannelUtils.showCreateOption(Constants.PRIVATE_CHANNEL, isAdmin, isSystemAdmin)) {
+        if (!ChannelUtils.showCreateOption(Constants.PRIVATE_CHANNEL, isTeamAdmin, isSystemAdmin)) {
             createPrivateChannelIcon = null;
         }