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)
