Fix tiny Tailwind config details (#82)

* Fix Tailwind darkMode CSS pref

See https://tailwindcss.com/docs/upgrade-guide#remove-dark-mode-configuration.

* Fix Tailwind 'purge' -> 'content'

See https://tailwindcss.com/docs/upgrade-guide#configure-content-sources.

* Remove additional cruft from Tailwind config

These literally weren't hurting anything but the upgrade guide
suggests toasting them, so might as well do it while I'm in here.

web/tailwind.config.js

@@ -1,9 +1,10 @@
 const defaultTheme = require('tailwindcss/defaultTheme')
 
 module.exports = {
-  mode: 'jit',
-  purge: ['./pages/**/*.{js,ts,jsx,tsx}', './components/**/*.{js,ts,jsx,tsx}'],
-  darkMode: false, // or 'media' or 'class'
+  content: [
+    './pages/**/*.{js,ts,jsx,tsx}',
+    './components/**/*.{js,ts,jsx,tsx}',
+  ],
   theme: {
     fontFamily: Object.assign(
       { ...defaultTheme.fontFamily },
@@ -18,9 +19,6 @@ module.exports = {
       },
     },
   },
-  variants: {
-    extend: {},
-  },
   plugins: [
     require('@tailwindcss/forms'),
     require('@tailwindcss/typography'),