Add jest-fetch-mock as dev dependency (#528)

Also add config/jest/setupJest.js so we can configure jest-fetch-mock

Test plan: I have verified that mocked fetch works as expected in a
downstream commit.

config/jest/setupJest.js

@@ -0,0 +1,3 @@
+// @flow
+
+global.fetch = require("jest-fetch-mock");

package.json

@@ -78,7 +78,8 @@
       "src/**/*.{js,jsx,mjs}"
     ],
     "setupFiles": [
-      "<rootDir>/config/polyfills.js"
+      "<rootDir>/config/polyfills.js",
+      "<rootDir>/config/jest/setupJest.js"
     ],
     "testMatch": [
       "<rootDir>/src/**/__tests__/**/*.{js,jsx,mjs}",
@@ -116,6 +117,7 @@
     "enzyme": "^3.3.0",
     "enzyme-adapter-react-16": "^1.1.1",
     "enzyme-to-json": "^3.3.3",
+    "jest-fetch-mock": "^1.6.5",
     "mkdirp": "^0.5.1",
     "static-site-generator-webpack-plugin": "^3.4.1",
     "tmp": "^0.0.33",

yarn.lock

@@ -56,6 +56,10 @@
   dependencies:
     seedrandom "~2.4.3"
 
+"@types/jest@^23.0.0":
+  version "23.3.0"
+  resolved "https://registry.yarnpkg.com/@types/jest/-/jest-23.3.0.tgz#5dd70033b616a6228042244ebd992f6426808810"
+
 "@types/node@*":
   version "9.4.7"
   resolved "https://registry.yarnpkg.com/@types/node/-/node-9.4.7.tgz#57d81cd98719df2c9de118f2d5f3b1120dcd7275"
@@ -3936,6 +3940,14 @@ jest-environment-node@^20.0.3:
     jest-mock "^20.0.3"
     jest-util "^20.0.3"
 
+jest-fetch-mock@^1.6.5:
+  version "1.6.5"
+  resolved "https://registry.yarnpkg.com/jest-fetch-mock/-/jest-fetch-mock-1.6.5.tgz#178fa1a937ef6f61fb8e8483b6d4602b17e0d96d"
+  dependencies:
+    "@types/jest" "^23.0.0"
+    isomorphic-fetch "^2.2.1"
+    promise-polyfill "^7.1.1"
+
 jest-haste-map@^20.0.4:
   version "20.0.5"
   resolved "https://registry.yarnpkg.com/jest-haste-map/-/jest-haste-map-20.0.5.tgz#abad74efb1a005974a7b6517e11010709cab9112"
@@ -5488,6 +5500,10 @@ progress@^2.0.0:
   version "2.0.0"
   resolved "https://registry.yarnpkg.com/progress/-/progress-2.0.0.tgz#8a1be366bf8fc23db2bd23f10c6fe920b4389d1f"
 
+promise-polyfill@^7.1.1:
+  version "7.1.2"
+  resolved "https://registry.yarnpkg.com/promise-polyfill/-/promise-polyfill-7.1.2.tgz#ab05301d8c28536301622d69227632269a70ca3b"
+
 promise@8.0.1:
   version "8.0.1"
   resolved "https://registry.yarnpkg.com/promise/-/promise-8.0.1.tgz#e45d68b00a17647b6da711bf85ed6ed47208f450"