Upgrade to Rails 6.0

[etagwerker]

This application is currently using Rails 4.2.

We should upgrade it to Rails 6.0 following this plan:

• Upgrade to Ruby 2.5 (Upgrade to Ruby 2.5 #57)
• Upgrade to Rails 5.0 (Upgrade Project to Rails 5.0  #83)
• Upgrade to Rails 5.1
• Upgrade to Rails 5.2
• Upgrade to Rails 6.0
• Upgrade to Rails 6.1