Restrict content to 'type/article', as specified by Wendy

"Content should display latest article (not multimedia or galleries)
from Content API"

https://jira.gutools.co.uk/secure/attachment/19165/iPad%20backpage%20-%20landscape.png

https://jira.gutools.co.uk/browse/IPAC-33
Prefer 'uk-news' as the Content API id for news

Wendy specifies "UK News" in her designs:

https://jira.gutools.co.uk/secure/attachment/19165/iPad%20backpage%20-%20landscape.png

https://jira.gutools.co.uk/browse/IPAC-33
Use UK for preference on all boxes, and don't show Tech twice

Wendy confirms that we want the UK-specific versions of News, Sport,
Technology and Comment (where they exist). Although the Guardian Edition
is sold in UK, USA, and Rest-Of-World editions, those are customised only
by advertising, with a single UK-specific version of content. Therefore,
we prefer the UK versions.

https://jira.gutools.co.uk/browse/IPAC-33?focusedCommentId=57980&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-57980
Update to latest version of content-api-client v2.7
Use stories with images if we can find them, preferring new stories

uk/commentisfree?show-elements=image&tag=type/article&page-size=13
Test for image not ready yet