Add integration test for locale linking behaviours

I refactored some of the specs to use some more shared context setup, and to
reduce the length of some of the examples which appeared. I found the previous
examples quite long and each example was actually testing several things. Since
I wanted to add yet another detail to a test scenario, I preferred to break it
up to simplify them and have each example cover only one particular aspect of
behaviour.
Include `available_translations` in links

Automatically generate a links list of `available_translations`. This is based
on grouping content items using the content_id and locale of each content item.