Add valuation comments relation at Budget Investment

Why:

Budget Investments already has an existing `comments` relation that is
on use. We need to keep that relation unaltered after adding the
internal valuation comments, that means scoping the relation to only
public comments (non valuation ones) so existing code using it will
remain working as expected.

A new second relation will be needed to explicitly ask for valuation
comments only where needed, again scoping to valuation comments.

How:

Adding a second `valuations` relationship and filtering on both
with the new `valuation` flag from Comment model.

app/models/budget/investment.rb

@@ -34,7 +34,10 @@ class Budget
 
     has_many :valuator_assignments, dependent: :destroy
     has_many :valuators, through: :valuator_assignments
-    has_many :comments, as: :commentable
+
+    has_many :comments, -> {where(valuation: false)}, as: :commentable, class_name: 'Comment'
+    has_many :valuations, -> {where(valuation: true)}, as: :commentable, class_name: 'Comment'
+
     has_many :milestones
 
     validates :title, presence: true