Customise branding at build time

[benfrancis]

(Suggested by KaiRo in #iot.)

As a developer I want to customise the branding of the gateway so that I can use my own brand on my own product based on the WebThings Gateway software.

Some places branding appears:

1. Logo during first time setup
2. Domain for tunnelling service
3. Logo in main menu

Note that the Wi-Fi SSID, default local host name, tunnelling service base domain and API endpoints are already configurable, but I believe the logo and base domain are currently hard coded in the UI.

Note that if people do start using the gateway software in their own products with their own branding in this way, we're going to need a policy about usage of the free WebThings cloud services.