This unfortunately required splitting the types into two. This led to
a lot of unfortunate code duplication that I didn't really enjoy
writing.
The new design allows users to pass an authentication token to make
webhook requests without the webhook token and allows to finally
edit the webhook channel.
The new design also uses a contextvar to store rate limiting
information so multiple instances or recreating instances no longer
clears the ratelimiting state since it's now essentially a "global"
object.
Closes#6525, closes#6662, closes#2509, closes#1761
Rapptz