Add optional beta banner

As some agencies haven't fully transitioned yet we want to show that the
main source for the documents is the agency website.

This commit adds the beta banner to be displayed if it's set to true and
if there is a beta message pass that in. It also tidies up some spacing
around the header.
Add beta banner and message for formats

Add flag and message to AAIB, DSU, MAIB, MSA and RAIB.
Merge pull request #71 from alphagov/beta-banner-for-documents

Beta banner for documents