Rails 6 upgrades (do not merge)

[nwalberts]

Existing work done in order to upgrade make_it_so generator to Rails 6

Unfortunately, the architecture of a Rails 6 app is different enough that I am putting this on pause

Things to investigate prior to continuing

• investigate using webpacker as new asset pipeline manager over sprockets, and what changes will be needed accordingly
• figure out if we want sprockets-rails 4 or to just leave it off and have all assets managed by webpacker
• work with consulting team on babel/webpack configuration to update the config that we add with make_it_so (we might be able to remove much of our old patterns now that webpacker comes standard with Rails 6)

And while we are at it

• remove shoulda configuration
• remove enzyme/karma configuration