Improve security of Nextcloud installation

[artis3n]

The following items are found in the below picture:

• Add 404 and 5xx error pages to signature.json to fix code integrity error
• Fix .htaccess file to prevent data directory from being directly accessible from the internet
• Add XSS protection headers to Nginx config
• Add Clickjacking protection headers to Nginx config
• Always serve the site over HTTPS - either through Let's Encrypt or self-signed cert
• Fix PHP OPcache settings

[artis3n]

XSS and Clickjacking headers are in nginx.conf, however Nextcloud is reading that those headers are not set. That needs to be investigated.

[artis3n]

PHP OPCache is set, but is not being picked up by Nextcloud.

[artis3n]

regarding htaccess:
https://www.nginx.com/resources/wiki/start/topics/examples/likeapache-htaccess/