/etc/apache2/apache2.conf - find the directory bits and make sure the /var/www one looks like:
<Directory /var/www/>
Options Indexes FollowSymLinks
AllowOverride All
Require all granted
</Directory>
sudo a2enmod rewrite
sudo a2enmod headers
Make sure all the site configuration files in /etc/apache2/sites-available have .conf on the end (especially if upgrading from previous Apache and it doesn't seem to be backwards compatible despite what the Linode library says).
Don't forget to sudo a2ensite all-the-sites.conf