Speculative fix for intermittently failing feature

Scenario in question is features/document-collections.feature:33

This scenario often fails on the step:
"And I see that "Feeding Wombats" is before "Wombats of Wimbledon" in the document collection"

I'm thinking that this might be because the post made in
"And I move "Feeding Wombats" before "Wombats of Wimbledon" in the document collection"
was aborted before completing by navigating straight to the preview page. If I'm right,
waiting for the loading spinner to go should fix this.
Merge pull request #1103 from alphagov/test-fixes

Speculative fix for intermittently failing feature

https://www.pivotaltracker.com/story/show/61376422