Remove half-width styles

I don't think these are used anywhere now.
Remove media queries from journey.scss

These don't actually target anything because:

1. It's not called a conversion-graph any more
2. The 2nd block isn't specific enough to ever be used
Refactor journey graph styles

Move the media query into the meat of the Sass, because it doesn't
actually need to be a media query. It's also not specific enough
to be used where it previously was.

Hide the tick line on the journey graph. It was never in any designs
so I'm not sure why it's there.

Add a tiny bit more space between the arrow and the journey graph.
Fix bug in journey styles: callout too wide

The callout under the journey graph is too wide, resulting in the edges
being cropped.

This commit makes the callout 1% smaller on both the left and the right,
and then moves the arrow 1% to the left to compensate (so it's still
centered horizontally).

Also add a bit more space to the top and the bottom, because it's
all quite bunched up.
Improve colour management in stylesheets

Use Sass variables to reference colours. Ideally we'd get
rid of the mixing I've added and just use standard greys,
but for now it's still nicer to use proper variables and
not just magic hex values.

Remove unused $primary-colour-text variable.

There shouldn't be any noticeable difference in the browser
as a result of this commit.
Move direct styles before extensions

When using an extension (ampersand), you generally want the styles
that apply to the element to be overridden by the thing you're
extending them with.

Use the cascade properly by applying the extended properties after
the properties that apply directly to the element you're targeting.
Use box-sizing mixin from frontend toolkit

The frontend toolkit provides this code, so we shouldn't
duplicate it here.
Switch README badges to SVG for hi-res funtimes

Also change from David to Gemnasium. We couldn't use it in the past
because we had a Gemfile, so it assumed this was a Ruby project. Now
it can tell that there's only JavaScript here.
Add Sass comments, switch scaleX to scale

By using scale rather than scaleX we can take advantage of a
mixin in the frontend toolkit.
Merge pull request #406 from alphagov/sass-improvements

Sass improvements