Allow users to block specific webhooks.

Users who choose to use certain accessibility tools (namely the pluralkit and tupperbox bots) for disorders such as DID or OSDD will send messages via a proxy. The bot then triggers a webhook to replace their original message. This becomes an issue when a user is trying to block someone. If you block the user who is using the proxy/webhook bots to speak, your block within a server is meaningless. All their webhook/proxy messages are still visible to you. It feels like a gray area of block evading. I have several users in a server I'm in blocked and I still have to see their proxied messages which is sometimes upsetting and I do not wish to leave the server because it generates me a fair amount of work (artist).