PLT 6125 Allow system admins to create users on closed servers (#5908)

* Update user.go

Added method to create user accounts using a privileged system_admin account

* Update user.go

Added respective method to handle the the APIv4 call to create users with privileged account(system_admin) by passing server restrictions of EnableUserCreation and/or EnableOpenServer set to false.

* Update user_test.go

Added test case

api4/user.go

@@ -72,6 +72,8 @@ func createUser(c *Context, w http.ResponseWriter, r *http.Request) {
 		ruser, err = app.CreateUserWithHash(user, hash, r.URL.Query().Get("d"), c.GetSiteURL())
 	} else if len(inviteId) > 0 {
 		ruser, err = app.CreateUserWithInviteId(user, inviteId, c.GetSiteURL())
+	} else if c.IsSystemAdmin() {
+		ruser, err = app.CreateUserAsAdmin(user, c.GetSiteURL())
 	} else {
 		ruser, err = app.CreateUserFromSignup(user, c.GetSiteURL())
 	}