Consider replacing sprockets with propshaft

[eoinkelly]

propshaft is a lighter and more modern replacement for sprockets.

The case for making this change

We use shakapacker for all CSS and JS processing so this should just be a drop-in replacement. There is nothing currently broken about sprockets but it seems like propshaft is the future so we should nudge devs away from relying on any of the features it provides that propshaft does not.

I tried this on a client project and it seemed to be as simple as just replacing the sprockets gem with propshaft.

The case against making this change

The official rails assets guide still uses sprockets so using propshaft might cause some confusion.

[lukeify]

Investigate whether either of these are needed. This is low priority for now.

[lukeify]

We have discussed removing sprockets entirely without a replacement in #508.