Highlight already-inserted tweets

Demo'd this to some VIPs and had some feedback that a visual indicator in the search results showing which bits of content that have already been embedded would be useful.

Start by tracing how the WordPress plugin renders search results and determines whether a tweet has already been embedded. Add a visible distinction for already-inserted tweets, then verify that embedded and unembedded results are clearly distinguishable.

Written by the indexing model from the issue text.