Add the following headers to email notifications (#830, #247):
* X-Redmine-Host: host name defined in application settings
* X-Redmine-Site: application title defined in settings
* X-Redmine-Project: identifier of the project that the notification is related to, if any
* X-Redmine-Issue-Id, -Author, -Assignee: ticket related info
* X-Redmine-Topic-Id: identifies the thread a message belongs to
