Improve output of `/ping` command

[TimJentzsch]

Some minor improvements to the /ping command:

• Use the same utility function has the other commands to display the server latency (get_duration_str)
• Add a message above the embed, similar to the other commands. It should include some sort of success indicator and the time it took to execute the command (e.g. 523 ms).
• Use the in-progress color (cyan) in the first message, before the server has been contacted. This should indicate that the comment hasn't finished executing yet.