Blacklight 7 & Rails 6 upgrade

Resources:

https://github.com/projectblacklight/blacklight/wiki/Update-to-Blacklight-7.0.1
https://github.com/projectblacklight/blacklight/wiki/Bootstrap-3-to-4-Migration-Guide

These sites helped with rails upgrades:

https://railsbump.org/
https://rubygems.org/
https://guides.rubyonrails.org/
https://railsdiff.org/
https://guides.rubyonrails.org/upgrading_ruby_on_rails.html
https://www.fastruby.io/blog/rails/upgrades/upgrade-rails-from-5-2-to-6-0.html