Added version drop-down toggle (that I will replace in a moment)

Author: bvaughn

src/components/LayoutHeader/Header.js

@@ -10,9 +10,9 @@ import HeaderLink from './HeaderLink';
 import Link from 'gatsby-link';
 import React from 'react';
 import {colors, fonts, media} from 'theme';
-import {version} from 'site-constants';
 import ExternalLinkSvg from 'templates/components/ExternalLinkSvg';
 import DocSearch from './DocSearch';
+import VersionToggler from './VersionToggler';
 
 import logoSvg from 'icons/logo.svg';
 
@@ -146,27 +146,7 @@ const Header = ({location}: {location: Location}) => (
               width: 'calc(100% / 6)',
             },
           }}>
-          <a
-            css={{
-              padding: '5px 10px',
-              whiteSpace: 'nowrap',
-              ...fonts.small,
-
-              ':hover': {
-                color: colors.brand,
-              },
-
-              ':focus': {
-                outline: 0,
-                backgroundColor: colors.lighter,
-                borderRadius: 15,
-              },
-            }}
-            href="https://github.com/facebook/react/releases"
-            target="_blank"
-            rel="noopener">
-            v{version}
-          </a>
+          <VersionToggler />
           <a
             css={{
               padding: '5px 10px',