PLT-7785: Slash commands can be issued to a channel in a team without it (#7567)

* Ensured that specified channel is a part of specified team

* Simplified approach to just infer team id from specified channel id to eliminate the attack vector entirely

model/client4.go

@@ -2808,7 +2808,10 @@ func (c *Client4) ListCommands(teamId string, customOnly bool) ([]*Command, *Res
 
 // ExecuteCommand executes a given command.
 func (c *Client4) ExecuteCommand(channelId, command string) (*CommandResponse, *Response) {
-	commandArgs := &CommandArgs{ChannelId: channelId, Command: command}
+	commandArgs := &CommandArgs{
+		ChannelId: channelId,
+		Command:   command,
+	}
 	if r, err := c.DoApiPost(c.GetCommandsRoute()+"/execute", commandArgs.ToJson()); err != nil {
 		return nil, BuildErrorResponse(r, err)
 	} else {