webmail: in initial start (sse) event, send the version, and ask user to reload if it changes

will prevent showing errors to users about new unknown fields that may be added
in the new version.

webmail/api.json

@@ -1358,6 +1358,13 @@
 					"Typewords": [
 						"string"
 					]
+				},
+				{
+					"Name": "Version",
+					"Docs": "",
+					"Typewords": [
+						"string"
+					]
 				}
 			]
 		},