Warn about outside senders, large sends and links that mislead

Four warnings, in Privacy & safety, and all of them start switched off.

That is not timidity. A client that begins by interrupting is one people
learn to click through, and a warning clicked through without reading
costs the same attention and buys nothing. The outside-sender warning
could not be on by default in any case: it measures against the domains
that count as yours, and with nothing configured every message in the
mailbox is from outside.

Your own identity domains are always internal and are not configuration.
An account signed in as [email protected] warning that example.com is
external would be absurd, and making it be typed in first is a foot-gun
that leaves the feature useless the moment it is enabled. Configured
domains are additional, and cover their subdomains -- matched on a dot
boundary, so example.com covers mail.example.com and not notexample.com,
which is exactly the domain somebody registers on purpose.

The four:

A banner names the sender's domain on a message from outside.

Sending outside names the outside recipients and asks, rather than
refusing. "This is going outside" is a rule and not something the sender
can check; a list of addresses is. It reads the full identity list rather
than the visible one, since hiding an identity from the From menu does not
make its domain somebody else's.

Sending to a large group asks once the count crosses a threshold, which is
what catches a reply-all onto a long thread. It counts people rather than
headers, so one address in To and nine in Cc is a message to ten.

Opening a link asks when the destination is not trusted, and always when
the link's text names one domain while its destination is another -- even
where that destination is trusted, because being trusted is not the same
as being the place the text claimed. On that mismatch the offer to trust
the domain is withheld: what would be trusted is the destination, and the
destination is not the thing in question. Anything that is not http or
https is left alone, since warning about a mailto: is noise and noise is
how a warning stops being read. Both bodies are covered, because a link in
a plain-text mail is linkified by us and points wherever it likes just as
readily as one the sender marked up.

The click is cancelled and the navigation re-issued after the answer,
since there is no way to hold a real navigation open across a dialog. The
reopen runs in the continuation of the dialog's own click, which is still
the gesture a popup blocker wants to see.

web/src/styles/app.css

@@ -1398,3 +1398,6 @@ button.dp-open:disabled { cursor: default; opacity: .5; }
 
 /* The senders whose remote images load without asking, in Privacy & safety. */
 .trusted-senders { display: flex; flex-wrap: wrap; gap: 6px; margin-bottom: 6px; }
+
+/* The banner naming a sender from outside the organisation. */
+.remote-banner.external-banner { background: var(--warn-soft); border-color: var(--warn); }