Allow monthly (dues based) memberships in Xtrulink

Allowing constituents to pay monthly (semi-annually, annually or quarterly) for a membership is a feature in Altru. It allows potential members to buy larger memberships without having to pay for the entire amount of a membership term all at once.

We may change gears and just make a special membership level that only lasts one month with the auto-renew feature.

We currently have another work-around that may work long term after some more testing, but having this built into Xtrulink would be much more preferred.