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.
* The descendants list for a comment was not being built correctly
* The root comments must be ordered “newest first”, but their
descendants must be ordered “newest last”
