Added greeting 'Hi {user display name},' to friendship notification email. As a side effect, user_mailer.friendship_notification.hi is not anymore reported as unused.
Use the _html suffix to mark that we expect the translations to contain links.
Unfortunately, we can't use the _html keys for the plain text emails, since the input options (e.g. username, url) will be html-escaped,
before it is passed to the view. So we need to use non-html-suffix keys for the plaintext views, in most cases. Only when the translation
options (e.g. url) are guaranteed to not contain any escapable characters can the same translation key be shared.
Fixes#2268.
Unfortunately there's still a lego translation when both halves of the sentence are joined together, but this at least reduces the number of combinations to translate.
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.
