Add and configure prettier code formatter

This commit adds prettier using the recommended configuration for
integrating with eslint:
https://prettier.io/docs/en/integrating-with-linters.html

I also added a .prettierrc.js file to configure our formatting based
on the existing .editorconfig.
https://prettier.io/docs/en/configuration.html

frontend/.eslintrc.js

@@ -14,7 +14,8 @@ module.exports = {
   extends: [
     'eslint:recommended',
     'airbnb-base',
-    'plugin:ember/recommended'
+    'plugin:ember/recommended',
+    'plugin:prettier/recommended' // last so it overrides other configs
   ],
   env: {
     browser: true

frontend/.prettierrc.js

@@ -0,0 +1,7 @@
+// prettier config
+module.exports = {
+    trailingComma: 'es5',
+    tabWidth: 2,
+    singleQuote: true,
+    endOfLine: 'lf'
+  };

frontend/package.json

@@ -91,13 +91,16 @@
     "ember-window-mock": "^0.5.4",
     "eslint": "^6.8.0",
     "eslint-config-airbnb-base": "^14.0.0",
+    "eslint-config-prettier": "^6.9.0",
     "eslint-plugin-ember": "^6.2.0",
     "eslint-plugin-import": "^2.19.1",
+    "eslint-plugin-prettier": "^3.1.2",
     "faker": "^4.1.0",
     "jsonwebtoken": "8.5.1",
     "loader.js": "^4.7.0",
     "mapbox-gl": "0.54.0",
     "pretender": "3.0.1",
+    "prettier": "^1.19.1",
     "rsvp": "^4.8.4",
     "sass": "^1.16.1",
     "semantic-ui-ember": "^3.0.0"