Remove incorrect Backdrop stub responses

There are some Backdrop stub responses in Spotlight that don't seem
to actually represent correct Backdrop responses and aren't used
by any of the dashboards we currently have.

They were discovered by @jcbashdown because they use the word
'bucket', which is currently being purged. So now seems like
a sensible time to just remove them entirely.
Fail gracefully with undefined data in bar_chart_with_number module.

If the valueAttr we're looking for is not found in the response,
supply null data instead. This will be displayed as (no data) in the
bar charts.
Add percentage option for bar_chart_with_number module.

If "format": { "type": "percent"} is set in config options,
display percentages throughout this module.

Expects data to be supplied from Backdrop between 0 and 1.

Add example to no-realistic-dashboard, and update tests.
Fix JSHint errors
Improve formatting logic in bar charts.

As suggested by @lennym in: https://github.com/alphagov/spotlight/pull/415#issuecomment-41658371

Use `format` option on collection, and use `this.format` consistently in view.
`this.format` uses the new formatters in `extensions/mixins/formatters.js`.

Notes:

  * Also extends journey collection to use format option, since the bar chart
    modules expect this option to be set.
  * Doesn't use `this.format` in `bar-chart.js` for numeric axes, since this produces
    odd results. Instead, falls back to our graph defaults for numeric axes, and only
    uses `this.format` for percent-formatted axes.
Make stylistic improvements.

Several code and stylistic improvements as suggested in this thread:
https://github.com/alphagov/spotlight/pull/420

Improvements:

* Better formatting in `spec.bar.js`
* Tidy code in `most-recent-number.js`
* In collection: pass entire options object through to axis, and allow
  user to specify a simple string rather than an object if desired
Extend formatters in journey module to use 3 s.f.

This now causes problems if the value is set to 0. Amend the
significant figures logic in the formatter to correctly return
0 when 0 is passed, and update tests.
Merge pull request #418 from alphagov/remove-unused-bad-backdrop-stubs

Remove incorrect Backdrop stub responses
Merge pull request #420 from alphagov/enhancement/add-percentage-support-in-bar-chart-with-number-module

Support percentages in bar_chart_with_number module