Allow device types to overridden in the config.

This feature currently only works well with our infrastructure’s varnish device detection.

Currently, tablets will display the desktop theme and apps will display the mobile theme. This feature allows you to specify a config setting to change that. So for example specifying `Garden.Devices.Tablet = ‘mobile’` will make tablets look like mobile devices on the server.
Merge branch 'hotfix/device-override' of github.com:vanilla/vanilla into stage