Disable and hide the Email link UI
There are a few commands for sending an email with the current page URL. See sendLinkForBrowser. This includes an "Email link" toolbar button, a response to an email keyboard button and an item under "File > Email link...".
I'm not sure if this actually does anything in practice, since it doesn't do anything on my system. Do we have an explicit protection for mailto:? If so, we should hide the corresponding UI.
Otherwise, we should consider disabling this feature because it can act as a proxy bypass. E.g. a content preview for the URL is loaded by the email provider. The menu items and toolbar button require an explicit user interaction, but can still be hit accidentally. Moreover, the keyboard binding could also be hit accidentally.
@dan it seems you upstreamed a patch to hide the "Share URL" item with MOZ_PROXY_BYPASS_PROTECTION, and this explicitly enabled the "Email link" item for macOS. Is there any particular reason why we deemed the "Email link" item safe, unlike "Share URL"?