Make sure that a favorite font is always set when using Glowing-Bear. 

The favorite font is set on startup and stored in local storage if none is set. On desktop, the favorite font is monospace; on mobile, it is sans-serif.
The default font can be overriden by the user at any time using the settings dialog

Fixes #378 
Related to #322 
I strongly oppose setting this to just `monospace` and would much rather have the old list of fonts used here, as a general catch-all font family is not only incontrollable for us, but it also might look really shit.