webmail: for replies/forwards, add button "send and archive thread" next to the "send" button, and give it a control+shift+Enter shortcut

the regular send shortcut is control+Enter. the shift enables "archive thread".
there is no configuration option, you'll always get the button, but only for
reply/forward, not for new compose.

we may do "send and move thread to thrash", but let's wait until people want it.

for github issue #135 by mattfbacon

webmail/api.json

@@ -1190,6 +1190,13 @@
 						"nullable",
 						"timestamp"
 					]
+				},
+				{
+					"Name": "ArchiveThread",
+					"Docs": "If set, thread is archived after sending message.",
+					"Typewords": [
+						"bool"
+					]
 				}
 			]
 		},