Subscription notification emails contain html entities

[roysmyth]

When posts are submitted from non-admin users, symbols and other characters are converted to html entities.

When a notification is sent to subscribers plain text email is used. The html entities show up in the post title and body sections of the email as codes not characters. This is easy to fix by using html_entity_decode($string, ENT_QUOTES) on both the post title and body before they are sent to wp mail.

[thebrandonallen]

Closing in favor of #2865