Improve quiz browsing

* Quizzes have additional fields:
  - Cover photo
  - Time per question
  - Privacy
* Students can now play all quizzes in the database,
  and search them with full-text searching.

Concerns #124
Fixes #91
Fix tooltips for buttons
Temporarily hide contact card for mobile
Redesign School's quiz list
Refactor
Fix quiz edit form styling
Fix additional info component
Fix tooltips in button groups
Position dropdown carets better
Fix tooltip for correct answer indicator
Update spacer classes
Fix button positions in quiz edit view
Use wells for no items state
Fix an error with viewing played quizzes
Change ID of quiz list to avoid conflicts
Style Student quiz listing
Add a scroll hover disabler
Mute endless scroll loading text
Add search icon to the search field
Fix flash messages