[KAIZEN-0] tvinger core-js til versjon 3.5.0

Gjøres pga følgende issues;
JakeChampion/fetch#750
JakeChampion/fetch#748
zloirock/core-js#751
zloirock/core-js#741

I bunn og grunn, core-js@^3.6.0 introduserte støtte for
String.prototype.split med regex som har sticky-flag. Pga bug her så
fungerer ikke funksjonen som forventet noe mer i eldre IE11 versjoner.

E.g `"test".split(/e/) === ['t', 'e', 's', 't']`, denne burde gitt
`['t', 'st']`

package.json

@@ -16,6 +16,7 @@
         "url": "git+https://github.com/navikt/modiapersonoversikt"
     },
     "scripts": {
+        "preinstall": "npx npm-force-resolutions",
         "start": "craco start",
         "build": "craco build",
         "test": "craco test",
@@ -39,12 +40,11 @@
         "@nutgaard/use-fetch": "^2.3.1",
         "@nutgaard/use-formstate": "^2.2.1",
         "classnames": "^2.2.6",
-        "core-js": "^2.5.7",
+        "core-js": "3.5.0",
         "craco-less": "^1.15.0",
         "detect-browser": "^3.0.1",
         "downshift": "^5.0.5",
         "faker": "^4.1.0",
-        "fetch-mock": "^6.5.0",
         "history": "^4.7.2",
         "js-cookie": "^2.2.0",
         "lodash.debounce": "^4.0.8",
@@ -137,13 +137,15 @@
         "eslint-config-prettier": "^4.3.0",
         "husky": "^2.2.0",
         "jest-enzyme": "^7.0.2",
-        "jest-fetch-mock": "^2.1.2",
         "jest-styled-components": "^7.0.2",
         "lint-staged": "^8.1.5",
         "prettier": "^1.19.1",
         "react-test-renderer": "^16.8.6",
         "typescript": "^3.8.0"
     },
+    "resolutions": {
+        "core-js": "3.5.0"
+    },
     "browserslist": {
         "production": [
             ">0.2%",

src/setupTests.ts

@@ -1,7 +1,7 @@
+import 'core-js/stable';
 import { configure } from 'enzyme';
 import EnzymeReactAdapter from 'enzyme-adapter-react-16';
 import 'jest-enzyme';
-import 'babel-polyfill';
 import 'jest-styled-components';
 
 configure({ adapter: new EnzymeReactAdapter() });