Upgrade Rails to version 6.1.x

[murny]

Upgrade should be pretty straight forward. Here is the release notes and upgrade guide:

• https://edgeguides.rubyonrails.org/upgrading_ruby_on_rails.html#upgrading-from-rails-6-0-to-rails-6-1
• https://edgeguides.rubyonrails.org/6_1_release_notes.html

However we should go back and use rails stuff instead of our homegrown code. As an example we wrote our own permanent URLs for activestorage blobs code a long time ago. Now this is included in Rails for free, so we should make the switch to use their version.

Also there may be other nice features included in this release we could look into. One example is lazy loaded images: rails/rails#38452