Upgrade to Rails 6.1

Rails 6.1 is released: https://edgeguides.rubyonrails.org/6_1_release_notes.html

This issue is for tracking the upgrade to Rails 6.1

There are other related issues that are labeled rails6.1

Gems that must be updated:

• devise-two-factor
• bullet
• default-value-for
• bundler-audit (0.6.1 requires low thor version, updating to 0.7.0.1 solves this problem)