CRM-15630 - ensure social media links work in email messages.

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* CRM-15630: Petition add signature thank you email doesn't send correct social networking links
  https://issues.civicrm.org/jira/browse/CRM-15630
Merge pull request #4594 from jmcclelland/petition-email-mode

CRM-15630 - ensure social media links work in email messages.