Add help link options to form errors

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Addresses #364301 (closed)

Sometimes we may want to include documentation links in model validation errors in order to give users more information about why the error is occurring and what actions they should take in order to resolve it. This change allows for help_page_url and help_link_text to be provided as ActiveModel options and be used to automatically append documentation links onto the end of the ActiveModel form errors. If help_link_text is not provided, 'Learn more.' will be used as the default text.

Example usage:

errors.add(
  :key,
  'cannot be used because it is known to belong to a compromised private key.',
  help_page_url: Rails.application.routes.url_helpers.help_page_url('security/ssh_keys_restrictions', anchor: 'compromised-keys')
)

This will produce the message:

Key cannot be used because it is known to belong to a compromised private key. Learn more.

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