Show type of alert in the alert banner

[andybroomfield]

Fix #345

• Support showing type of alert in the banner template
• Show the type of alert in the banner by default

What does this change?

Provides a default space in the template to show the output of type of alert. This uses a value from manage display, so it will only show if configured to do so. This will be the default in new installs only.

How to test

1. To fresh install
2. Choose a non localgov_base derived theme (localgov_base overrides the template).
3. Create an alert banner of each type
4. View alert banner, which should now show the type of alert.

How can we measure success?

Fixes accesbility issue identified in #345 , allbeit in this case without the role as its just plain text.

Have we considered potential risks?

This might not be the right solution, it won't effect current installs, it won't override existing themes.
However in this case it's more about providing an accessible default, as the issue identified in #345 is likley the themes responsibility?

Images

Accessibility

• Tested with screen reader
• Navigable with keyboard
• Colour contrast passed
• The change doesn't use only colour to convey meaning

[andybroomfield]

Are the names:

• Announcement
• Minor alert
• Emergency
• Death of a notable person
  Suitable for public display? They where only intended for internal use.