Surface installed extensions when known, and consider using them for clustering

[jgraham]

We collect some/all installed extensions with each report. We know that extensions can be the cause of compatibility issues. Therefore the dashboard should:

• Have some way to see installed extensions for each report
• Add extensions as additional data to the clustering
• Where a bucket has a high prevalence of a particular extension being installed, add that to the summary of the bucket along with the domain and the example of a typical report.