Discord: prevent 403s to avoid rate limiting

[blueridger]

Hitting 403s on the discord API gives your API key a worse reputation, potentially causing worse rate limits. The discord fetcher should be changed to check if the api key has permission to access a channel before fetching from it, and skip that channel proactively if it does not have permission.

https://github.com/sourcecred/sourcecred/blob/main/packages/sourcecred/src/plugins/discord/fetcher.js

https://discord.com/developers/docs/topics/rate-limits#:~:text=IP%20addresses%20that%20make%20too,%2C%20403%2C%20or%20429%20statuses.