PLT-6139 (Server): Private Channel member managing (#5941)

Adds an EE policy feature to allow restricting system-wide which level of
Admins can manage the membership of private channels.

utils/config.go

@@ -288,6 +288,7 @@ func getClientConfig(c *model.Config) map[string]string {
 	props["RestrictPrivateChannelManagement"] = *c.TeamSettings.RestrictPrivateChannelManagement
 	props["RestrictPublicChannelDeletion"] = *c.TeamSettings.RestrictPublicChannelDeletion
 	props["RestrictPrivateChannelDeletion"] = *c.TeamSettings.RestrictPrivateChannelDeletion
+	props["RestrictPrivateChannelManageMembers"] = *c.TeamSettings.RestrictPrivateChannelManageMembers
 
 	props["EnableOAuthServiceProvider"] = strconv.FormatBool(c.ServiceSettings.EnableOAuthServiceProvider)
 	props["GoogleDeveloperKey"] = c.ServiceSettings.GoogleDeveloperKey