gfm.css is no longer supported

[t3chguy]

warning gfm.css@1.1.2: Package no longer supported. Please use github-markdown-css instead.

[t3chguy]

This brings visual changes

Before

After

Likely to be delegated to Compound

[Johennes]

@nadonomy how do you feel about us going ahead with this for now? It'll be a while until we get to touching the timeline as part of the Compound adoption work and would be nice to eliminate the unsupported dependency in the meantime.

[nadonomy]

Wfm. We can catch any nasty unintended consequences on develop.