Force host and port on file server level for external files

[Nicklas Nordborg]

It might be useful to be able to set a specific host and port on a FileServer item which will override the host/port on the URL of a File item linked to that file server. The use case is that a storage solution may use a "public" part that is entered as the url for a file, and an "internal" part that is used when the file is accessed via the BASE server.

Hmm... just wondering if we should have support for path re-writing as well. Eg.

http://public.server/path/to/file

becomes

http://internal.server:port/internal/path/to/file

This might be a bit more complex... so unless someone has a good idea to implement this in a good way let's start with the host/port override.

[Nicklas Nordborg]

(In [5618]) Fixes #1596: Force host and port on file server level for external files