WIP new asset pipeline

[marlo-longley]

Alrighty this works but def still a WIP and absolutely breaks older versions of the app usage
This also badly breaks tests.
Next steps is to fix all this but wanted to preserve the work online.

Major changes:

• use propshaft and importmaps
• use Blacklight 8
• Add a package.json file within the generator template directory so that we could get leaflet, blacklight-frontend, and bootstrap JS directly from NPM
• vendorize and basically adopt the leaflet-sidebar-rails js and styles and stop using the gem (was https://github.com/mejackreed/leaflet-sidebar-rails)
• turn the leaflet-sidebar-rails sass into css
• make minor inline edits to other vendor js (well, actually the only cases were adding let before a variable assignment) -- a mix of old and new JS syntax that should probably be looked at more deeply
• convert the blacklight_heatmaps sass into css (was only 10 lines)
• reorganize blacklight_heatmaps JS
• set up a fake stimulus controller to test that setup
• Delete an application.html.erb file from blacklight_heatmaps that I don't think did anything
• Delete generator template files from blacklight_heatmaps that I don't think did anything
• add a weird/bad _head.html.erb file so that we can include css and vendor js that does not extend leaflet's L

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working with this code in a generated test app....

[jcoyne]

probably delete this file