Allow user to specify additional Monitor addresses directly in plugin

What if multiple people should be notified about site downtime? It would be useful if there were a way to provide additional notification addresses (you could do opt-in e-mails to each, to comply with various laws), right in the plugin.

Start by tracing the plugin's existing Monitor notification flow and how the primary recipient is configured. Clarify how additional addresses are entered, whether each recipient opts in, and what behavior counts as complete, including the relevant privacy and notification requirements.

Written by the indexing model from the issue text.