Add 'unreleased version' & 'old version' warning banner to non-latest versions of docs

[munnerz]

trafficstars

To make it clear to users when they are looking at unreleased or old versions of the docs, we should include a banner at the very top of the page/under the header bar indicating to the user that the version they are looking at is not 'current'.

I'm not sure what the best-practice way to do this is, we could either hook into the versioning system/plugin configuration or otherwise string match on the URL that the user is viewing.