Add support for tag: `page:tags`

[adamjohnson]

How to reproduce

1. Start hammer: hammer-start
2. Navigate to the backpage template in Brand Patterns: http://localhost:2000/brand-patterns/views/backpage.html
3. Open the Backpage template in the brand-patterns repo in your code editor then add <r:page:tags /> anywhere in the template.

Tag to add support for

page:tags:

<h2><r:page:tags /></h2>

Link to tags in CleanSlate repo

cleanslate/app/lib/slate/radius/tags/page.rb (Github)