billing: Extract get_seat_count function.

This is a simple generalization of get_latest_seat_count and is useful
for calculating "what will be the realm's license count if this
number of (guest) users is added?" without duplicating any of the math
logic. Will be used in the next commits.

corporate/lib/stripe.py

@@ -58,14 +58,23 @@ STRIPE_API_VERSION = "2020-08-27"
 
 
 def get_latest_seat_count(realm: Realm) -> int:
+    return get_seat_count(realm, extra_non_guests_count=0, extra_guests_count=0)
+
+
+def get_seat_count(
+    realm: Realm, extra_non_guests_count: int = 0, extra_guests_count: int = 0
+) -> int:
     non_guests = (
         UserProfile.objects.filter(realm=realm, is_active=True, is_bot=False)
         .exclude(role=UserProfile.ROLE_GUEST)
         .count()
-    )
-    guests = UserProfile.objects.filter(
+    ) + extra_non_guests_count
+    guests = (
+        UserProfile.objects.filter(
             realm=realm, is_active=True, is_bot=False, role=UserProfile.ROLE_GUEST
         ).count()
+        + extra_guests_count
+    )
 
     # This formula achieves the pricing of the first 5*N guests
     # being free of charge (where N is the number of non-guests in the organization)