Make Milestones general, and not specific to Budget Investments

Generalize the Budget::Investment::Milestone model to a
polymorphic Milestone model so it can be used for entities
other than Budget::Investment.

config/locales/en/activerecord.yml

@@ -10,7 +10,7 @@ en:
       budget/investment:
         one: "Investment"
         other: "Investments"
-      budget/investment/milestone:
+      milestone:
         one: "milestone"
         other: "milestones"
       milestone/status:

config/locales/es/activerecord.yml

@@ -10,7 +10,7 @@ es:
       budget/investment:
         one: "Proyecto de gasto"
         other: "Proyectos de gasto"
-      budget/investment/milestone:
+      milestone:
         one: "hito"
         other: "hitos"
       milestone/status: