Filter out characters that are already in attendance from the character dropdown

[Lemmings19]

This can probably be applied in other parts of the site too.

Hey mate, not sure if there was already a feedback on this so I am sorry for the spam... Nice improvement would be when I am creating a raid after a raid night and I import players from warcraftlogs... I still need to add people who were on bench or didnt show up at all... when I click to add another player it shows you the full list instead of those that are missing... this doesnt make much sense as you cant add the same player twice. Not sure if this is hard or easy to implement but would save a lot of time . Anyhow, keep up the great work!

https://discord.com/channels/732998376798552210/733019346649088121/923130103070883840