Membership form is unclear about what membership is

[abrandemuehl]

Right now, the sign in form causes some confusion about "membership" really is. It starts with "Joining ACM is completely free", but people who haven't paid can't login, which is confusing.

This is kind of in parallel with #127, but has a different focus.

If the plan is to eventually allow unpaid members to log in, then we should have a stopgap solution that just clarifies the wording of the form.

[abrandemuehl]

Specifically the distinction between "joining" and "official membership" should be discussed.

[narendasan]

This is being addressed with the next release (50years and ad service) but @tommypacker make sure to provide the correct wording.