Add "npm audit" command, fix errors

This does not fix errors that stem from the nightwatch dependency, as
those vulnerabilities are only corrected in a beta version of Nightwatch
that isn't ready for prime time. The good news is that, as a
devDependency, nightwatch is not installed on Heroku.

.eslintrc.extra.js

@@ -59,5 +59,10 @@ module.exports = {
         'react/display-name': 'off',
         'react/prop-types': 'off',
         'react/no-unescaped-entities': ['error', {forbid: ['>', '}']}],
+
+        // Temporarily work around this bug:
+        // https://github.com/evcohen/eslint-plugin-jsx-a11y/pull/437
+        'jsx-a11y/heading-has-content': 'off',
+        'jsx-a11y/anchor-has-content': 'off',
     },
 };

.gitignore

@@ -6,6 +6,7 @@
 # testing
 /coverage
 selenium-debug.log
+selenium-server.log
 
 # generated files
 /build
@@ -18,12 +19,12 @@ selenium-debug.log
 .env.production.local
 
 npm-debug.log*
+
+# We used Yarn in the past
+yarn.lock
 yarn-debug.log*
 yarn-error.log*
 
-# We use yarn.lock, not package-lock.json
-/package-lock.json
-
 # Just an extra precaution. At the moment, some demo instances of this site have
 # a data/ directory which should not be published.
 data/

README.md

@@ -14,20 +14,20 @@ Facebook.
 
 ### Install
 
-1. [Install Yarn](https://yarnpkg.com/lang/en/docs/install/)
-2. Run `yarn`
+1. [Install Node and NPM](https://nodejs.org/en/download/)
+2. Run `npm install`
 
 ### Run
 
-Run `yarn start`
+Run `npm start`
 
 ### Test
 
-Run `yarn test`
+Run `npm test`
 
 ### Analyze
 
-To analyze the size of the JavaScript bundle that will be served, run `yarn
+To analyze the size of the JavaScript bundle that will be served, run `npm run
 size`.
 
 ### Notes