Update milestones translatable fields

Note the title field was hidden since commit 01b9aa8, even though it was
required and translatable. I've removed the required validation rule,
since it doesn't seem to make much sense and made the translatable
tests harder to write.

Also note the method `I18n.localize`, which is used to set the
milestone's title, uses `I18n.locale` even if it's inside a
`Globalize.with_locale` block, and so the same format is generated for
every locale.

spec/models/budget/investment/milestone_spec.rb

@@ -9,9 +9,9 @@ describe Budget::Investment::Milestone do
       expect(milestone).to be_valid
     end
 
-    it "is not valid without a title" do
+    it "is valid without a title" do
       milestone.title = nil
-      expect(milestone).not_to be_valid
+      expect(milestone).to be_valid
     end
 
     it "is not valid without a description if status is empty" do