Define DEFAULT_FRESHLY_CLOSED_LIMIT in the Note model to allow the 7 day limit to be referenced in the API controller and in the new `freshy_closed_until` logic. The default value is `7.days`. API users can still override this, but the website uses that default for the duration of the green "freshly closed" notes markers.
It's a convention in rails to name your mailers with a Mailer suffix, and is also common to name the class after the recipient (e.g. User, Admin). So UserMailer seems a reasonable choice.
Now that we have all api controllers inheriting from a common base,
it's easier to override the deny_access handler without having to
switch between both.
Fixes#2064
