Add environment indicators

* Include environment based favicons
* Set environment_style and environment_label at deploy time
* Include a development only default environment
* When no environment is set, show no indicator
* Styles pulled from publisher:
https://github.com/alphagov/publisher/pull/203
Allow apps to override default favicons

* Use `:favicon`
Namespace images

* Prevent clashes with images from parent apps (eg favicons)
Include details about environment indicators
Prevent order of tests affecting test result

* Be explicit about what we are expecting in the no environment test
Merge pull request #17 from alphagov/distinguish-environment

Distinguish between environments