Disocrd channels improvements

[L-e-x-o-n]

• Discord channels configs were removed from Discord site configs at teiserver/admin/site, they are now set at http://localhost:4000/admin/discord_channels (which is not a new thing but it was only used for a few channels before) Make sure to transfer any missing channel IDs from the old to new configs
• All channel IDs are now read from the the new config when sending messages
• Replaced usage of Nostrum.Api with Teiserver.Communication where possible Teiserver.Communication uses Nostrum.Api in the background. By using Teiserver.Communication everywhere it is easier to replace Nostrum.APi if it breaks in the future