Display flash messages at the top if there are any.

@ourmaninamsterdam we are using the same markup and style as the opt-out bar at the moment but we need to update this.
[WIP] Add HTML for global alert
Remove layout containers to allow the alert to fit full width
Add Global alert heading flag, modifier class and <p> formatting
Add global-alert to style guide
Merge pull request #460 from moneyadviceservice/feature/global-alerts

Feature/global alerts
Puts optimizely behind a feature switch.
Corrects readme info about feature detection.
Don't memoize categories in fake repository

CategoryTreeReader#build_tree deletes attributes as it traverses the hierarchy
Add hover state to search icon submit button
Merge pull request #463 from moneyadviceservice/search_icon_hover_state

Add hover state to search icon submit button
Merge pull request #461 from moneyadviceservice/f/optimizely-toggle

Puts optimizely behind a feature switch.
Give non-navigation categories a "Home" breadcrumb
Add feature coverage for non-navgational category breadcrumbs
Add cassettes for non-navigational category breadcrumb scenario
Merge branch 'non-navigation-category-breadcrumbs'
Add outline to header logo
Add hover state to search button
Move skip to content link
Style focus state of header home link