Standardize our fonts throughout Dagit. Makes things look a lot nicer.
- Use the default font-family supplied by Blueprint, with -apple-system as the primary choice. For recent MacOS versions, this should be San Francisco.
- Define a standard monospace font-family for usage everywhere, Source Code Pro. We use this in some places, but there is a fairly loose mixture of this override and the regular monospace default, which in most cases is sadly Courier New.