Why:

Budget Investment's valuators need to be able to comment on investments
without making those comments public. We need a way to clearly make a
distinction to avoid "leaking" internal valuation comments.

How:

Adding a boolean `valuation` attribute defaulted to false to the Comments
table, and index on it with concurrent algorithm as explained at
https://robots.thoughtbot.com/how-to-create-postgres-indexes-concurrently-in

The name `valuation` was chosen instead of `internal` because of the more
specific meaning as well as avoiding a collision with existing internal_comments
attribute on Budget::Investment model (soon to be deprecated & removed)

Why:

Internal valuation comments are only for admins and valuators,
not for the public view.

How:

Adding a `not_valuations` scope and use it at the `public_for_api` one

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.

Why:

Budget Investment's valuators should be able to see internal valuation
comments thread at both show and edit views.

How:

At Valuation::BudgetInvestmentsController:
* Include CommentableActions to gain access to the entire feature, with
required resource_model & resource_name methods.
* Add the only possible order (oldest to newest)
* Load comments on both show & edit actions, passing `valuations` flag
to the CommentTree in order to only list those.

At CommentTree:
* Use `valuations` flag as instance variable to decide wich
comment threat to load: valuations (if relation exists) or comments.

How:

Using a local variable at partials to set a hidden true/false value for
`valuation` parameter on the comment creation form.

Allowing that new param at the comment controller and using it when
building a new Comment.

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

As Budget::Investment has two relationships over commentable polymorphic
relationship, the counter_cache is counting the sum of both comments and
valuations.

We don't show valuations count anywhere, only the (public) comments so
we just use comments.count in this case