Summary: Misc small Dagit fixes - #3343, #2568, #1015

Summary:
Remove unused theme CSS, improve comment color in default theme #3343

When parent (eg: solids) is missing in config, only show required children as missing #2568

Fix go-to-line-start to account for whitespace, cannot fix autocompletion display easily #1015

Test Plan: Fiddle with codemirror unfortunately

Reviewers: dish

Reviewed By: dish

Differential Revision: https://dagster.phacility.com/D5630