Add banner when viewing previous versions

[frangio]

Some users are visiting the docs for older versions of a project when they are actually interested in the latest version. We can have a banner at the top of older versions reminding people that they're not looking at the latest version.

[frangio]

This should probably be added in article.hbs using something like:

{{#unless (eq @root.page.componentVersion.version @root.page.component.latestVersion.version)}}
<!-- banner -->
{{/unless}}