Improve thumbnail load times

[masaball]

IndexedFiles must be reindexed for this work to have an effect. Without reindexing all thumbnails will display as the missing content icon.

Improvements needed:

• Populate with icon data instead of asset_path when item is missing thumbnail
• Because of being in the term frequency method, the work as it stands breaks the browse page.
  • Move thumbnail logic out of term frequency method and into its own method
  • Or rework the checks and search logic so that the term frequency method runs without causing an error when solr_parameters[:q] is missing.
• This work probably needs an explicit fallback for when a video is missing a thumbnail or content_ss is otherwise unable to be pulled from solr.

Investigation needed:

• Does this affect overall search load times? Need to test with larger datasets.
• Ensure that the thumbnails are returned for both browse and search, and that this doesn't break other routes that run through the search builder/catalog controller.
• Is there a way to return the thumbnail data without needing a reindex?

[cjcolvar]

This is great work, but I'm unsure if it is the best solution given the need to reindex and I'm wondering if data-uris might end up being more of a problem than an improvement. I'd like to leave this PR in draft and talk over how it compares to alternative solutions.