Modern asset handling

[anderscarling]

Asset handling in godmin is really old, mostly using the pre-rails-5.1 (see #222) method of pulling in javascript and css through gems.

This method makes it somewhat hard to isolate the versions of dependencies used by godmin from those of the main app (if both uses gems for assets), even though godmin most likely uses a separate .css and .js file. In some cases it also has us stuck on old versions of some dependencies.

This old tech stack can also interfere with users desire to run a more modern stack (see brunoprietog's comments in #265.)

It would be great to find a way to modernise the asset handling in a fashion that allows us to pull in the latest versions of dependencies through yarn/npm - but also isolate the way assets are handled in godmin from what our host application will be using. Maybe we should just ship complied and minimised css and js with the gem?