Add missing period.
Bug 845904: Completely remove welcome messages

See also bug 865054.
Bug 845904: Handle languages not in our list

If user's language is one that's not in our list of languages that
we support for newsletters, e.g. 'pt-BR' when our list has 'pt',
we need to be sure our form displays their language. Detect this
case and add their language to the field's choices.
Bug 845904: Fix test after removing signup message
Bug 845904: Email is read-only

Since user cannot currently change the email address, don't display
it as a form field. Just show their email address on the page as
text.
Merge branch 'readonly-email' into bug-845904-newsletter-preference-center