All of these have been deprecated for years, if we're releasing a new
major version, let's take the opportunity to do some cleanup.
* Remove deprecated `:bypass` option from `sign_in` helper,
use `bypass_sign_in` instead.
* Remove deprecated `devise_error_messages!` helper,
use `render "devise/shared/error_messages", resource: resource` instead.
* Remove deprecated `scope` second argument from `sign_in(resource, :admin)`
controller test helper, use `sign_in(resource, scope: :admin)` instead.
* Remove deprecated `Devise::TestHelpers`,
use `Devise::Test::ControllerHelpers` instead.
Closes#5739
The deprecation of `devise_error_messages!` wasn't super clear on what
was happening and how to get rid of the message, not it has a more
detailed explanation with an example of what to look for and what to
replace it with.
Closes#5257.
