External Downtime Message

Creates a JSON response which phpBB.com (and other sites) can check and display.
* Active - If 1 then it will display on other sites, if 0 then it won't
* Name - Used to make it dismissable, this can be used as a cookie name
* Content - The message content, must be plaintext
* URL - Link to the status site for more information.

In addition this adds a config table which needs adding to local installs. For some basic test fixtures see `src/phpBB/StatusSiteBundle/Resources/config.sql`. This should be kept updated.

`src/phpBB/StatusSiteBundle/Resources/doc/ConfigVariablesGuide.md` should be update. It is a doc file for all possible config values.