Add a graphs controller.

[connorshea]

• Adds better_errors gem
• Adds the ability to run USE_TEST_DATA=true bundle exec rake db:seed in order to run the application in development mode with the test data.
• Changes the default value for the browser columns in the feature table to nil instead of '{}'
• Adds graphs pages

TODO:

• The graph values don't quite add up when there are more complex browser support objects, e.g. it handles { version_added: true } but I'm not 100% sure it handles { version_added: true, prefix: '-webkit'} and it definitely doesn't handle support objects that are arrays of these hashes.

[connorshea]

I don't want this PR to become too complicated, so I'm going to merge it now. The results on the graphs page aren't entirely representative, but it's good enough for now.

I'll add proper tests in a separate PR, maybe using factories?