Corrective action: Danger comment for new validations

A recent production incident exposed the fact that we are able to add new validations to models without confirming the impact they have on existing records.

For example, we added a validation on the users website_url to ensure this value is a valid URL containing https:// or http://, but there was no mechanism in place to identify existing records which would then become invalid.

This is a corrective action to create a Rubocop rule which will automatically flag new validations that touch existing columns - this comes from @stanhu and @engwan here

We decided to add a comment by the danger bot instead #340144 (comment 1466228003).