Tokens don't conform to `Authenticatable`

[0xTim]

In Vapor 4, Tokens don't currently conform to Authenticatable. An example of where this would be useful is when invalidating a token:

func logoutHandler(_ req: Request) throws -> EventLoopFuture<HTTPStatus> {
  let token = try req.auth.require(UserToken.self)
  return token.delete(on: req.db).transform(to: .ok)
}

Currently the only ways to achieve this are with extra DB calls, but the middleware has already done that lookup, so the token should be cached somewhere where it can be retrieved

[tanner0101]

Fixed in #672