add thumbnail of src

[mfrederickson]

Use wkhtmltoimage to generate thumbnail of src for tiles in concerto, if it is installed. Use imagemagick to shrink them to tile size. Need to figure out how and where to store them, and then link to them like the remote video one does.

[mfrederickson]

Store in media table in concerto. Refresh as needed via delayed job.

[mfrederickson]

Here's another possible option for a screenshot of the src to evaluate. https://github.com/sindresorhus/pageres