add button to set seen flag on front

back/validator/schemas/setFlag-schema.json

@@ -0,0 +1,20 @@
+{
+    "type": "object",
+    "properties": {
+        "mailboxId": {
+            "type": "number"
+        },
+        "messageId": {
+            "type": "number"
+        },
+        "flag": {
+            "type": "string"
+        }
+    },
+    "required": [
+        "mailboxId",
+        "messageId",
+        "flag"
+    ],
+    "additionalProperties": false
+}