Create valuation comments creation ability

Why:

Only admins or valuators (for those investments they've assigned) can
create internal valuation comments on them.

How:

* Creating a new `comment_valuation` ability for admins and valuators in
the same manner the `valuate` ability works.

* Adding a validation at Comment model for those with `valuation` flag
active that checks if the author can make a valuation comment on the
commentable, as well as the respective active record error messages.
This will prevent comments from being created at a controller level as
well.

* Improving comment factory trait `valuation` to have an associated
investment, author that is a valuator and setting the valuator on the
valuators list of the investment

app/models/abilities/valuator.rb

@@ -5,8 +5,8 @@ module Abilities
     def initialize(user)
       valuator = user.valuator
       can [:read, :update, :valuate], SpendingProposal
-      can [:read, :update, :valuate], Budget::Investment, id: valuator.investment_ids
-      cannot [:update, :valuate], Budget::Investment, budget: { phase: 'finished' }
+      can [:read, :update, :valuate, :comment_valuation], Budget::Investment, id: valuator.investment_ids
+      cannot [:update, :valuate, :comment_valuation], Budget::Investment, budget: { phase: 'finished' }
     end
   end
 end