Jhbate/allow multi invites

Allow a configurable number of days that we insist on between invites. This prevents spamming of invites but still allows the user who has removed someone from their team to add them back at a later date.

@kentquirk 
This probably isn't the best way to do this. The time library has a Duration object, and you could construct a Duration object and call the Hours method with daysAgo*24, then subtract that from the current date to get the right value. That will be more reliable than the code you have here.