Metadata description for tag pages

[alinaboghiu]

In tag manager it's possible to specify optionally a tag "description". This ends up in the page html as:
<meta name="description" content="...">

We think that when this description is not specified there should be a default value.

In DCR the default value is Latest news, sport, business, comment, analysis and reviews from the Guardian, the world's leading liberal voice

TODO

• Confirm with Peter Martin what the ideal wording for the default tag should be
• Ensure the meta tag is picked up in Google searches (this is not currently the case, possibly to do with the encoding)

Note

In Frontend this default value is missing altogether, but since we're already at 10% DCR tag pages and we expect to be at 100% before the end of the Q we are not going to fix this in Frontend.