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 pull request #2351 from vanilla/hotfix/device-override

Allow device types to overridden in the config.