i18n: Initialize FormatJS.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

package.json

@@ -11,6 +11,7 @@
     "@babel/preset-env": "^7.5.5",
     "@babel/preset-typescript": "^7.3.3",
     "@babel/register": "^7.6.2",
+    "@formatjs/intl": "^1.9.7",
     "@giphy/js-components": "^4.3.1",
     "@giphy/js-fetch-api": "^2.4.0",
     "@uppy/core": "^1.7.1",
@@ -19,6 +20,7 @@
     "autoprefixer": "^10.0.0",
     "autosize": "^4.0.2",
     "babel-loader": "^8.0.6",
+    "babel-plugin-formatjs": "^10.2.6",
     "blueimp-md5": "^2.10.0",
     "cache-loader": "^4.0.0",
     "callsites": "^3.1.0",
@@ -84,6 +86,7 @@
   },
   "devDependencies": {
     "@babel/eslint-parser": "^7.11.3",
+    "@formatjs/cli": "^4.2.6",
     "@types/clean-css": "^4.2.2",
     "@types/css-minimizer-webpack-plugin": "^1.1.3",
     "@types/jquery": "^3.3.31",