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

config/locales/en/activerecord.yml

@@ -296,6 +296,10 @@ en:
             image:
               image_width: "Width must be %{required_width}px"
               image_height: "Height must be %{required_height}px"
+        comment:
+          attributes:
+            valuation:
+              cannot_comment_valuation: 'You cannot comment a valuation'
       messages:
         record_invalid: "Validation failed: %{errors}"
         restrict_dependent_destroy:

config/locales/es/activerecord.yml

@@ -292,6 +292,10 @@ es:
             image:
               image_width: "Debe tener %{required_width}px de ancho"
               image_height: "Debe tener %{required_height}px de alto"
+        comment:
+          attributes:
+            valuation:
+              cannot_comment_valuation: 'No puedes comentar una evaluación'
       messages:
         record_invalid: 'Error de validación: %{errors}'
         restrict_dependent_destroy: