Send resends and releases to the right recipients

[jszobody]

Fixes #31.

Resend and Release both copied the recorded Cc and Bcc lists onto a new To built from the clicked row. That sent mail to the wrong people.

Resend

Before: resending the Bcc row of an email to a@ (To), c@ (Cc) and b@ (Bcc) sent To b@, Cc c@, Bcc b@. So c@ got the email again, and b@ appeared twice.

After: a resend goes only to the recipient on that row. This matches the dashboard prompt, "Resend this email to {address}?", and the dashboard's suppression check, which only looks at that one address. To reach several original recipients, resend each row.

Release

Release marks every sibling row as sent, so it has to deliver to the whole original envelope.

Before: releasing from a Cc or Bcc row put that address in To and left out the original To recipients, though their rows still flipped to "sent".

After: release sends the original To, Cc and Bcc lists, whichever row you release from.

The release confirmation is the browser's native confirm() dialog, which a page screenshot can't capture. Its text now names every recipient:

- Release this sandboxed email and send it for real to cc@example.com? This cannot be undone.
+ Release this sandboxed email and send it for real to to@example.com, cc@example.com? This cannot be undone.

Changes

• ResentMessage sends to the row's address only.
• ReleasedMessage sends the recorded To, Cc and Bcc. A row with no recorded envelope falls back to its own address.
• New EmailMessage::envelopeAddresses() supplies the Release prompt.
• The readme and docblocks now describe resend as single-recipient.

Tests

• A resend of each of the To, Cc and Bcc rows reaches only that address, in To.
• A release from a Bcc row sends the full original envelope.
• The release prompt lists every recipient.

All 455 tests pass, and static analysis is clean.

#16 (duplicate resent tags) changes the same class and has its own PR.