When loading entries, if the feed to which an entry belongs is not in the scope, load it via Ajax from the server.

Before this commit, a reload of all feeds was triggered. Now only the missing feed is loaded, which is less resource intensive.
When lazy-loading images, if an entry image did not originally have a valid "src" attribute, no lazy-loading is attempted, and no spinner placeholder is displayed.
Fixed acceptance test to account changes in the way feeds are refreshed.