Don't hide 'is_recur' option on Contribution Pages

Apart from the fact that we want to support one-off contributions as
well soonish, hiding the recur option has the undesirable side effect of
also making the recur frequency settings unavailable -- but we need
these. (See issue #44)

While some organisations might want to enforce only monthly
contributions, such customisations should be implemented locally for
these organisations (or with a new option in core) -- it's not something
to be dictated by the Payment Processor...