Validate Newsletter segment_recipient value

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

config/locales/en/activerecord.yml

@@ -261,6 +261,10 @@ en:
             attachment:
               min_image_width: "Image Width must be at least %{required_min_width}px"
               min_image_height: "Image Height must be at least %{required_min_height}px"
+        newsletter:
+          attributes:
+            segment_recipient:
+              invalid: "The user recipients segment is invalid"
         map_location:
           attributes:
             map:

config/locales/es/activerecord.yml

@@ -257,6 +257,10 @@ es:
             attachment:
               min_image_width: "La imagen debe tener al menos %{required_min_width}px de largo"
               min_image_height: "La imagen debe tener al menos %{required_min_height}px de alto"
+        newsletter:
+          attributes:
+            segment_recipient:
+              invalid: "El segmento de usuarios es inválido"
         map_location:
           attributes:
             map: