Update MIGRATION.md
BUGFIX: re-enable CDN js debugging in a robust way

May be disabled if needed via site setting
BUGFIX: show usernames logic, logic failure
making the color customizer a little nicer with dark themes
Merge pull request #2361 from awesomerobot/master

making the color customizer a little nicer with dark themes
Merge pull request #2357 from techAPJ/master

Update MIGRATION.md
shorten "all subcategories"