[dagit] Remove custom fonts
We don't really need our custom web fonts at this point, and they break aggressively in the --path-prefix case.
- We don't really use Open Sans at all now for normal text, in favor of using system fonts.
- For monospace, it's fine to just use system fonts as well. Safari uses SF Mono, which is great. Other browsers can fall back to system fonts as needed. I put Source Code Pro on the list before Menlo because it still looks better than Menlo, but honestly Menlo is fine too.
Test Plan: yarn build-for-python in Dagit directory, run dagit with a path prefix. Verify that monospace fonts render properly without falling back to anything terrible (Courier).
Reviewed By: bengotow
Differential Revision: https://dagster.phacility.com/D6983