Add `Run pending migrations` button

[elalemanyo]

Rails 6 added ActiveSupport::ActionableError module to define actions we want perform on errors, right from the error page.
PendingMigrationError added by default a button that says Run pending migrations. Clicking on this button would dispatch rails db:migrate action. Page will reload once migrations run successfully.
Would be great to have this button also when we use better_errors.

[RobinDaugherty]

This is a duplicate of #443. A great idea, but nobody has had the time to implement it yet.