Adding favicon auto generation to the build process

Concerns:

1.) This only applies in builds, i.e. gulp serve does not run the favicon task since we serve the index.html page.
2.) This happens after HTML minification, meaning that we end up with HTML that is mostly on a single line, but with one block of HTML before the closing head element.

I haven't tested this on devices yet, but looking at the HTML it seems good to go. Happy to do the testing when I get into the London office (likely to be tested on Thursday).

In the mean time - any comments?