Added more configurable parameters to shared milstone config

no issue

- Instead of having those values hard coded, make them quickly configurable
- Those values are deciding if we send an email or not, the milestones will be created either way:
    - minDaysSinceImported: the amount of that days should've passed since the last member was imported
    - minDaysSinceLastEmail: minimum amount of days that need to pass
    - maxPercentageFromMilestone: the max percentage that the current value can be away from the achieved milestone (e. g. 999 members should not send and email for 100 members)

ghost/core/core/shared/config/defaults.json

@@ -213,6 +213,9 @@
                     "values": [100, 1000, 10000, 50000, 100000, 250000, 500000, 1000000]
                 }
             ],
-            "members": [100, 1000, 10000, 25000, 50000, 100000, 250000, 500000, 1000000]
+            "members": [100, 1000, 10000, 25000, 50000, 100000, 250000, 500000, 1000000],
+            "minDaysSinceImported": 7,
+            "minDaysSinceLastEmail": 14,
+            "maxPercentageFromMilestone": 0.35
         }
 }