starboard improvements

[danthe1st]

This PR contains the following improvements to the starboard:

• Users are now fully excluded from their own starboard messages
• The check for "invalid" users is removed from the reaction removal because the user is always null for these events (we don't use user caching)
• Users reacting to their own messages are now generally excluded from the star count (though these reactions should normally be removed by the voting regulation listener)
• Users can now react with any of the configured star emojis
• The thresholds for using different star emojis in the starboard message are reduced from 10 and 25 to 5 and 10 respectively