Make Cards.valid_response? private
Use 'create!' instead of 'create' in seeder
Put breadcrumbs on grid
Navbar is now responsive

Make navbar responsive. Change styling on 'Get Started' button
for responsive layout.
Implement average performance indicator on cards

Add average method to CardSubmissionLogs and serializer. Implement
logic on Backbone logs object to hold state. Implement render
Handlebars helper for display.
Add serializer from last commit
Call to action for 'Practice' if the user has cards
Clarify card queue in dashboard
Remove confusing array challenge
Make practice callout more obvious
Move label to left of name in lesson show

Move label for greater clarity. Modify styling for consistent button
size and remove callout.
Add explanatory text to practice
Add explanatory text to challenge summary

Add explanatory text to challenge summary and put all summary pages
on the grid.
Remove back buttons
Change button styles for better clarity
Tweak style on assignment show view

Tweak styling and make instructions more explicit in seeder.