Add issue subscription check to API (#10967)

close #10962

Adds `GET /api/v1​/repos​/{owner}​/{repo}​/issues​/{index}​/subscriptions​/check`
 -> return a `WachInfo`

templates/swagger/v1_json.tmpl

@@ -5217,6 +5217,53 @@
         }
       }
     },
+    "/repos/{owner}/{repo}/issues/{index}/subscriptions/check": {
+      "get": {
+        "consumes": [
+          "application/json"
+        ],
+        "produces": [
+          "application/json"
+        ],
+        "tags": [
+          "issue"
+        ],
+        "summary": "Check if user is subscribed to an issue",
+        "operationId": "issueCheckSubscription",
+        "parameters": [
+          {
+            "type": "string",
+            "description": "owner of the repo",
+            "name": "owner",
+            "in": "path",
+            "required": true
+          },
+          {
+            "type": "string",
+            "description": "name of the repo",
+            "name": "repo",
+            "in": "path",
+            "required": true
+          },
+          {
+            "type": "integer",
+            "format": "int64",
+            "description": "index of the issue",
+            "name": "index",
+            "in": "path",
+            "required": true
+          }
+        ],
+        "responses": {
+          "200": {
+            "$ref": "#/responses/WatchInfo"
+          },
+          "404": {
+            "$ref": "#/responses/notFound"
+          }
+        }
+      }
+    },
     "/repos/{owner}/{repo}/issues/{index}/subscriptions/{user}": {
       "put": {
         "consumes": [