The gateway should send the banner and accent_color fields for users

[MinnDevelopment]

Currently, you can only get these fields through the GET/users/:user_id endpoint. This makes it nearly impossible for a moderation bot to keep track of these fields, since it requires to fetch each and every user it sees.

An alternative is to add previous state to the update event, which would solve this problem as well. I have an existing feature request for this with #3272

[GoldenAngel2]

Duplicate of #3577

[MinnDevelopment]

Not a duplicate since I ask for the entire gateway.