Generate thumbnails of interactive plots on results page for faster page loading

[Bezbakri]

Implements #319

Checklist

Please check all that apply to your proposed changes

• HTML code change
• Added package dependency
• Added data
• Changed django model
• Documentation change
• Added or changed TaskRunner

[manning-ncsa]

As of commit 440dd75 the workflow-based thumbnail generation is working! The JPEG formatted images generated are on the order of 200KB, and repeated page loads in my dev browser show that this image is cached. The initial page load is ~1MB and subsequent page refreshes are ~400KB. In constrast, v1.7.3 page loads are ~12MB each time you load the page; no caching appears to be occuring.

Along the way I noticed that the "Transient information" task never actually did anything and so I excised it, adding a db migration to remove all its existing TaskRegister objects.

I also incorporated the image cropping (formerly known as "trimming") into the workflow as a task, so the workflow now looks like:

A comparison of a random transient with the current release Blast v1.7.3 shows good agreement, although it is not obvious why the local aperture is tighter in the dev version:

Blast v1.7.3:

Dev: