add the ability to use local cached fonts, per bug 1274955

Minimal QA:
- run `make devel`.  Start chrome and open the inspector's network tab.  Open the GUI.  Verify that you are not loading any files from Google, but are loading fontfamily.css from the local files.  Verify that the charm browser tabs, the onboarding help, and the inspector look fine.
- run `make prod`.  As before, but verify that you are now loading font CSS from Google.
- run `make devel`.  Verify that the browser looks OK in Firefox too.

Note that on Ubuntu, the fonts are not actually downloaded because Ubuntu includes them.

Preferred additional QA:
- Try on IE 10, Mac OSX Chrome, and Mac OSX Firefox.  Verify that the fonts are downloaded as needed and that things look OK.
- Try Safari too.

Super-duper awesome QA:
- Deploy this in a charm and do the same browser tests.

Note that I thought that we would use local fonts by default.  However, in my experience, and unsurprisingly, the Google CDN has much better speed, and the difference is quite noticeable and problematic.  Therefore, I expect that the charm will default to CDNFonts: true, and charm configuration will have to turn it off explicitly.

Thank you!