Add `ActiveModel::Errors#add_on_present` method. Adds error messages to present attributes.

This will render the error message on :#{attribute}_confirmation instead
of on attribute itself. When rendering confirmation errors inline on the
form with form builders such as SimpleForm and Formtastic it is
confusing to the ender user to see the confirmation error message on the
attribute element. Instead it makes more sense to have this validation
error render on the confirmation field instead.

The i18n message has been updated for the confirmation validator error
message to include the original attribute name.

Revert "Fixed Issue #2884 ActiveModel::SecurePassword code / rdoc conflict; an elucidated explanation has been provided and ActiveModel en-locale updated to present a less confusing attribute name for 'password_digest' in form errors due to "has_secure_password""

This reverts commit ddbea947.

Reason: Includes non-doc changes.

Fixed Issue #2884 ActiveModel::SecurePassword code / rdoc conflict; an elucidated explanation has been provided and ActiveModel en-locale updated to present a less confusing attribute name for 'password_digest' in form errors due to "has_secure_password"

Remove weak_passwords list and the length/strong password validator, leave that up to the programmer

Signed-off-by: NJosé Valim <jose.valim@gmail.com>