webmail: show link to webaccount interface in top right

only if account web interface is enabled on the same listener and same http/https scheme.

webmail/api.json

@@ -1492,6 +1492,13 @@
 						"Settings"
 					]
 				},
+				{
+					"Name": "AccountPath",
+					"Docs": "If nonempty, the path on same host to webaccount interface.",
+					"Typewords": [
+						"string"
+					]
+				},
 				{
 					"Name": "Version",
 					"Docs": "",