feat: support for IE11 browser

frontend/package.json

@@ -13,6 +13,7 @@
     "ace-builds": "^1.4.7",
     "clipboard": "^2.0.4",
     "core-js": "^3.9.1",
+    "css-vars-ponyfill": "^2.4.3",
     "js-base64": "^2.5.1",
     "lodash.clonedeep": "^4.5.0",
     "lodash.throttle": "^4.1.1",
@@ -27,7 +28,8 @@
     "vue-lazyload": "^1.3.3",
     "vue-router": "^3.1.3",
     "vuex": "^3.1.2",
-    "vuex-router-sync": "^5.0.0"
+    "vuex-router-sync": "^5.0.0",
+    "whatwg-fetch": "^3.6.2"
   },
   "devDependencies": {
     "@vue/cli-plugin-babel": "^4.1.2",
@@ -64,6 +66,6 @@
   "browserslist": [
     "> 1%",
     "last 2 versions",
-    "not ie <= 8"
+    "not ie < 11"
   ]
 }