Email the post content in the email.

For forum subscriptions and post/thread mentions, the actual content of the post is emailed to the recipient.
Split emails and dashboard notification content into separate files

There is now an optional separate view file for dashboard notifications.  This allows for different content to be sent in the email than is displayed on the dashboard.
Merge pull request #13 from deweller/feature/email-notification-content

Email the post content in the email.