Bump govuk_frontend_toolkit to 0.20.0

This is to pick up the new media() forms.
Import campaign and tour styles from static application

These originally lived in the static application (for legacy reasons)
but the respective views live in the frontend application. Rather than
have this separation it's easier to keep the styles with the frontend
app so there are fewer dependencies.
Remove dependency on media-down in campaigns

We should be styling things as mobile UP, not desktop DOWN. This gets
us one step closer to having pages that follow the responsive mantra
correctly.
Remove dependency on media-down mixin in tours stylesheet

Inverted the styles for this view so that they are now mobile-first.
Remove media-down mixing from Frontend application
Move search styles into the Frontend application

These were previously in the static app, but as the views live in the
Frontend application they're better off together.
Remove search-meta-data mixin and darken the grey breadcrumb
Replace grey with equivalent semantically named colour

Where possibly we should be following the service manual:
https://www.gov.uk/service-manual/design-and-content/resources/colour-palettes
Merge pull request #343 from alphagov/move-styles-from-static-to-frontend

Add stylesheets for frontend views across from static