[WIP] Add flag to bypass two-step authentication for subscriptions

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

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This section details on the original issue you should resolve

<issue_title>Add a new flag to bypass two step authentication for subscribe/unsubscribe to pad</issue_title>
<issue_description>Hi, I would like to add a flag called 'ActivationEmailRequired' that we could set up in the settings.json. If the flag is set to 'No', for subscribing and unsubscribing to a pad, etherpad will not send an email with an activation link but directly subscribes the user to pad upon entering the email address and hitting the subscribe button. Shall I implement this feature?

Thanks!
</issue_description>

Comments on the Issue (you are @copilot in this section)

@JohnMcLear @mani89 sounds good, go for it :) @JohnMcLear I set this as enhancement but it might actually be a legal requirement to verify an email address prior to sending just to avoid spamming... I think it's required...