Completely remove usage of sprockets or propshaft

[westonganger]

Why:

• Sprockets v4 manifest file has all sorts of rough edges with rails engines
• Exposing the engines assets publicly can give away details about things that should be internal only
• We can easily avoid it altogether and just load the assets as part of the application layout
• There are such minimal amount of asset code that there really is no benefit to any HTTP caching