[dagit] Upgrade CodeMirror, fix error rendering

Summary:
A minor upgrade to CodeMirror, but mostly this fixes a bug with how errors are displayed. A CodeMirror change introduced a new way of setting classes on lint marks, so we just need to reflect that in our own code.

Relevant CM change: https://github.com/codemirror/CodeMirror/commit/18aa69e17cc7703f106fbe03992456b8e59e8cdc#diff-01477ffc2e6f10be0f276fcf2243cc3312b9165b00775ebccf7daf937fcef522

Test Plan: View Playground in Dagit with errors, verify that backgrounds render correctly.

Reviewers: bengotow, sashank

Reviewed By: sashank

Differential Revision: https://dagster.phacility.com/D5136