Faster file view via CodeMirror

This milestone tracks the delivery of enabling a rewrite of the blob view using CodeMirror. One of the key goals of this milestone is to speed up the core workflow.

Slack message about an initial POC for this work https://sourcegraph.slack.com/archives/CHXHX7XAS/p1655738402276249

All issues for this milestone are closed. You may close this milestone now.