Avoid a format validation security warning

This was actually a false positive, since our new regular expression
does the exact same thing. However, false positives generate noise and
make it harder to deal with real issues, so I'm changing it anyway.

We could add a more advanced regular expression, like
`URI::MailTo::EMAIL_REGEXP`. However, this expression marks emails with
non-English characters as invalid, when in practice it's possible to
have an email address with non-English characters.

app/models/newsletter.rb

@@ -3,7 +3,7 @@ class Newsletter < ApplicationRecord
 
   validates :subject, presence: true
   validates :segment_recipient, presence: true
-  validates :from, presence: true, format: { with: /@/ }
+  validates :from, presence: true, format: { with: /\A.+@.+\Z/ }
   validates :body, presence: true
   validate :validate_segment_recipient