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
add editCode support to studio (playlab)
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.
Added some mobile styling.
Moved outside of "Banner" so that banners on csedweek would show up on mobile.
Better fix for only loading pattern images that are available.
Merge pull request #455 from code-dot-org/csedweek-mobile-banners

Csedweek mobile banners