Contract list now scales with spawned vessels (#4172)

## About The Pull Request

As it says in the title, outpost contracts will now scale per vessel
in-sector by an addition of 5, with some coding help from Zimon (I did
something incredibly stupid)

## Why It's Good For The Game

You remember those rounds you had, where you had to share 15 contracts
with close to 10 or more vessels in-round? This makes it so you won't
have worry about having to share 15 contracts for a round with a high
vessel count.

## Changelog

:cl:
balance: The outpost contract limit will now scale with the amount of
vessels in-round
/:cl:

code/modules/overmap/objects/outpost/outpost.dm

@@ -38,7 +38,7 @@
 
 	/// The maximum number of missions that may be offered by the outpost at one time.
 	/// Missions which have been accepted do not count against this limit.
-	var/max_missions = 15
+	var/max_missions = 10
 	/// List of missions that can be accepted at this outpost. Missions which have been accepted are removed from this list.
 	var/list/datum/mission/missions
 	/// List of all of the things this outpost offers
@@ -141,6 +141,7 @@
 	return "[person_name] [pick(GLOB.station_suffixes)]"
 
 /datum/overmap/outpost/proc/fill_missions()
+	max_missions = 10 + (SSovermap.controlled_ships.len * 5)
 	while(LAZYLEN(missions) < max_missions)
 		var/mission_type = get_weighted_mission_type()
 		var/datum/mission/M = new mission_type(src)