[FIX] Calculate prorated costs correctly; use same userlib function to calculate and add to database the expiry date for group memberships based on days as for anniversary groups - this avoids losing a period when adding periods and solves daylight savings time issues; avoid "skipping" months when calculating date intervals
[ENH] Allow for use of PayPal subscription interface when within parameters (keeps shipping from being charged in case shipping has been set up on the PayPal account); Better descriptions on the payment form