Switch to using terser instead of babel

Babel has this bug which appeared after trying to update to the latest version, and remained even after rolling back to an older version.  Extremely annoying.  babel/minify#974
Add grunt-terser.
Remove babel, core-js, lodash, which was used just once in build-popup.js, and react-virtualized, which wasn't used at all.
Remove package-lock.json from .gitignore and commit it, to make it easier to track exact package changes.

.gitignore

@@ -2,5 +2,4 @@
 /build/rjs
 /build/out
 /node_modules
-/package-lock.json
 /release

Gruntfile.js

@@ -199,14 +199,7 @@ module.exports = function(grunt) {
 			}
 		},
 
-		babel: {
-			options: {
-				shouldPrintComment: (val) => /@license|@preserve|\/\*!/.test(val),
-				comments: false,
-				minified: true,
-				compact: true,
-				sourceMaps: false
-			},
+		terser: {
 			rjs: {
 				files: {
 					"build/out/js/common/error-handler.js": "src/js/lib/error-handler.js",
@@ -362,7 +355,7 @@ module.exports = function(grunt) {
 			`cleanupManifest:${target}:${env}`,
 			"buildPopup",
 			"requirejs",
-			"babel",
+			"terser",
 			"sync:build",
 			"clean:rjs"
 		]);

build/scripts/build-popup.js

@@ -2,7 +2,6 @@ const ReactDOMServer = require("react-dom/server");
 const React = require("react");
 const requirejs = require("requirejs");
 const fs = require("fs");
-const _ = require("lodash");
 
 
 const ConfigPattern = /({[\s\S]+})/m;
@@ -13,7 +12,6 @@ const configFile = fs.readFileSync("./src/js/require-config.js", "utf8");
 const currentPopupHTML = fs.readFileSync("./src/popup.html", "utf8");
 const currentReactMarkup = currentPopupHTML.match(RootPattern)[1];
 const match = configFile.match(ConfigPattern);
-let config;
 
 
 if (!match) {
@@ -31,11 +29,11 @@ navigator = {
 };
 
 	// we have to wrap the bare object in parens to eval it
-config = eval("(" + match[1] + ")");
+const config = eval("(" + match[1] + ")");
 
 	// switch some of the libraries to mocked versions so that we can require
 	// the App component outside of a browser
-config.paths = _.assign(config.paths, {
+config.paths = Object.assign(config.paths, {
 	cp: "../../build/mock/cp",
 		// the mock tracker needs to create a window global and then require
 		// the original tracker module