Add mailer for user MFA authentication app updates

[jenshenny]

Is your feature request related to a problem?

Currently, emails are sent if a user adds or removes a Webauthn security device https://github.com/rubygems/rubygems.org/pull/3695.

It would be nice to send an email if someone enables / disables a MFA TOTP device too.

Webauthn device mailer previews for reference:

• adding a device
• removing a device

[jenshenny]

@garyhtou is going to work on this!