Why:
A Newsletter can only be sent if the are available user recipient emails
and that means the `segment_recipient` value actually corresponds to a
function on the UserSegments class.
We could rely on the UserSegments::SEGMENTS constant as the list of
possible user segments functions that a Newsletter can use to gather
emails, so any value not included in that hash would not be valid.
But to be 100% sure the newsletter can get a recipients_list we should
just check if the UserSegments class has a method with same name as the
`segment_recipient` value.
How:
* Adding an validation method that checks if UserSegment has a method
with same name as the `segment_recipient` value.
* Adding an scenario to the Newsletter model spec to check this
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