Add CI

[stevepolitodesign]

This commit adds a CI workflow through GitHub actions, and is inspired by capybara_accessibility_audit.

Because CI runs against a matrix of Ruby versions and Rails versions, some special considerations needed to be made.

• The Gemfile.lock was removed and ignored to avoid noisy diffs when working locally.
• Logic was added to the Gemfile to load the desired version of Rails, and account for some bugs with Rails 6 and certain versions of Ruby.
• Conditional logic was placed in a few files (migrations, application record) and the schema was updated to account for differences between versions

A future commit will introduce a setup script and set the default version of Ruby.