Allow team admins to specify how long members have to renew

[rww]

Currently, in teams which allow members to renew their own membership, members have a week from the time they're notified that they need to renew until their expiration. This is a rather short amount of time for some teams, especially those with users that aren't frequently online. Therefore, an option for team admins to customize how long the window is for members to renew would be useful.