Our seat count calculation is different for guest user than normal users
(a number of initial guests are free, and additional marginal guests are
worth 1/5 of a seat) - so these checks we apply when a user is being
invited or signing up need to know whether it's a guest or non-guest
being added.
This commit changes the invite API to accept invitation
expiration time in minutes since we are going to add a
custom option in further commits which would allow a user
to set expiration time in minutes, hours and weeks as well.
Mateusz Mandera
Zixuan James Li
Sahil Batra
Anders Kaseorg