Updates to models, CSS to integrate support for new question types, and other small fixes

[sequoiatree]

• Switched from CSS to LESS to reduce boilerplate
• Used flexbox to display ranges (i.e. sliders) nicely on mobile devices in portrait mode
• Added handlers for serving custom pages on HTTP 404 and 500 errors (#18)
• Implemented configuring quantitative questions to use the number input type, an alternative to the range input type
• Implemented option questions, which allow the respondent to pick a nonnumeric element of a closed list (i.e. not an open text field)
• Implements toggle button behavior for skipping questions (#84)
• Added modal on the personal information page to warn the user that their submission is final (#77)
• Added back buttons that are locale-aware (#66)
• Added all nontrivial source files to the linter (#38)
  • As a consequence of the build passing the linter, all nontrivial code have docstring documentation (#12)
• Refactored and streamlined the client-side API (#80)
• Implemented a custom template tag (localize_url) for generating locale-dependent URLs correctly
• Renamed AJAX endpoints to be locale-independent
• Reduced template boilerplate by using composition

[jonathan-j-lee]

Why was #60 closed? You can add commits on top of an open PR.

[jonathan-j-lee]

Update: I've brought this branch into a usable state, but have branched off to:

1. Convert the CSS into LESS (improves maintainability with patterns such as selector inheritance).
2. Fix the styling to work on mobile devices.

Once styling is complete, I will merge that branch into sequoia, at which point sequoia can be reviewed and merged into master.

[jonathan-j-lee]

Ran these changes by @JustinMi.