Add no robots header AAIB Report urls

Before we do anything stop crawlers hitting aaib-reports urls, or anything else in our exclusion list for that matter!
Create AbstractDocument class from CmaCase class

As with the other apps lets abstract common document code into an AbstractDocument class to make it easier to inherit from this when creating other types of documents.
Allow the finder to deal with AaibReports

Instead of simply using the same presenter for all documents, we can switch based on the document_type attribute and use this to specify custom metadata attributes for document types we know about.
Make date labels visible on finder document listing

Previously we were hiding all labels with css. Instead lets add a special class for dates so we can target the label and display them to the user.
Use the correct noun depending on document_type

Instead of hardcoding the document_noun to cam cases use the document_noun from the schema
Use custom mappings for attribute names
Move metadata formatting to helpers

Move document metadata formatting into helper methods and remove the complexity from the view.
Move time formatting to i18n
Merge pull request #38 from alphagov/feature/add-aaib-reports

Allow handling of AAIB Reports