This commit adds a Strict-Transport-Security header to
the nginx config file generated by the nextcloud module.
The Strict-Transport-Security header is recommended in
official guide for hardening Nextcloud installations:
https://docs.nextcloud.com/server/16/admin_manual/installation/harden_server.html
Further, if it is not set, we see a warning in the security scan results
in the Nextcloud admin panel:
```
The "Strict-Transport-Security" HTTP header is not set to at least "15552000" seconds. For enhanced security, it is recommended to enable HSTS as described in the security tips
```
4590a2380e