Collapse the navbar on mobile devices

When accessing exercism.io from a mobile device, the navbar wraps around
the screen and doesn't look nice. Make it look better by using
bootstrap's collapse plugin and collapsing the navbar when on mobile
devices.
Merge pull request #1664 from mattr-/fix-navbar-on-mobile

Prettify the navbar when browsing on mobile devices.
Trigger dependent destroys on submission
Add cleanup task for orphan comments
Add recognition for nimrod exercises
Use path to determine track and slug. See #1663
Add redirect for legacy setup pages
Update show.erb
Merge pull request #1669 from cmckni3/patch-1

Fixes exercise done button class
Adds a Code of Conduct & Link in Contributing Docs

adds link to CoC in contributing documentation
Merge pull request #1671 from meaganewaller/adds-coc

Adds a Code of Conduct & Link in Contributing Docs
Fetch schema when doing seeds
Tweak seed data
starts creating the initial onboarding pages
adds progress bar to all onboarding steps
fill in the fourth step when you've commented
remove unnecessary z-index to avoid dropdown menus from being hidden by progress bar
Merge pull request #1662 from meaganewaller/progress-bars

starts creating the initial onboarding pages
Include migration history in seeds

This will allow people to run the db:migrate task locally without having
it blow up.
Tweak seed task