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
Remove redundant views
Fixes after refactoring views
Restructure HTML to reflect the new layout
Connect js collapse function to the styleguide page
Fix the layout on the child category page
Rename child and grnadchild categories to parent and child
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