[Finishes #82526660] add reference area containing instruction videos to non-Blockly puzzle types (Multi, Match, etc)
Update multi.en.yml
Add -p switch to runner.rb for running UI tests in parallel

Usage: `./runner.rb -p 3` will run UI tests 3 at a time.
Fix artist level attempting to load 'undefined' as a default pattern image.
Only attempt to load snowflake stamp for Anna/Elsa levels.
Switch from threads to processes so ENV vars aren't shared
Go back to prior sharing images.
add editCode support to studio (playlab)
correct CSS for hiding reference area
Added pages for listing special events.
Deleted old special events page.
Updated special event pages.
Added link to Special Events page in events navigation.
fix pd teachers report
Merge branch 'staging' of github.com:code-dot-org/code-dot-org into staging
Merge pull request #453 from code-dot-org/editCode_studio

add editCode support to studio (playlab)
remove broken link from public/private
Merge branch 'staging' of github.com:code-dot-org/code-dot-org into staging
multiple course homepage redesign, certificates, gallery pagination
Merge pull request #449 from code-dot-org/fixstampload

Only attempt to load snowflake stamp for Anna/Elsa levels.