fix(notifications): drop the sender's name from their own chat message

[bmc08gt]

A chat notification reads "Some Person: whoa" directly under a line that already says Some Person. The server composes the body that way because a plain notification has nowhere else to put the sender, but MessagingStyle does — it draws the Person above the message in a group, and in a DM the notification's own title carries the name. So the name lands twice wherever a sender is attached.

planMessageBody strips the prefix in applyChatStyle before style.addMessage.

Two things it is deliberate about:

• It matches every name the sender is known by, not the one being rendered. Those disagree: the line is drawn with the address-book name when there is one, while the server composed the body from the profile's display name. Matching only the displayed name would leave the prefix in place exactly for the senders the user has saved as contacts.
• It only strips an exact name match. A message that happens to start "Neil: " is left alone unless Neil sent it, and a body that is nothing but the prefix is kept rather than posted empty.

senderName is now the first non-blank of that same list, which is the chain it already resolved through.