Downtime manager

www.agileplannerapp.com/boards/173808/cards/8805

As a content designer
I want a better way to indicate on start pages when a service is down for planned maintenance
So that I don't have to re-edition multiple pages and push these through workflow, then repeat the process again once the downtime is over

related:
https://github.com/alphagov/govuk_content_models/pull/277
https://github.com/alphagov/govuk_content_api/pull/209
https://github.com/alphagov/frontend/pull/709

![screen shot 2014-12-23 at 10 47 42](https://cloud.githubusercontent.com/assets/319055/5536936/3269fbd4-8a91-11e4-988c-a23ad9cb213f.png)
![screen shot 2014-12-23 at 10 47 29](https://cloud.githubusercontent.com/assets/319055/5536935/3268c6ec-8a91-11e4-87d1-a6cb2bbf8de6.png)

Publisher uses a pattern: `flash[:success]`, `flash[:warning]` or `flash[:error]` to be more explicit about the nature of the message. See https://github.com/alphagov/publisher/pull/302 for details.