Limit active Agent token count to 5

We had a discussion about the agent design today and there was concern about making a lot of api calls to obtain status information for multiple tokens

At some point, we talked about limiting Active tokens to 5 to help with the setup.

@gitlab-org/configure does anyone have any issues/concerns with limiting the number of Active tokens an agent can have? We'd still allow for mutiple revoked tokens.

Thanks!

/cc @hfyngvason