Optionally specify base URL for public links

If your OC server is behind a NAT device, public links must not use the internal hostname but the public name. Right now, there is no way to specify the public base URL.

This patch allows you to define the base URL for public links.

E.g. your server has the IP 192.168.1.2 and uses the internal name "owncloud.internal.example.com". Obviously, public links pointing to "owncloud.internal..." cannot be used from the internet. Therefore, you define a DNAT rule on your gateway, making your server available as "gateway.example.com:8080". To make all public links contain this URL, you specify "https://gateway.example.com:8080" as the "public base URL" under administrative setting.