ZipStream for subalbums

[oxpa]

Hello.

First of all, thank you for the gallery and subalbums. It's totally awesome!

There was a problem with my albums: they tend to occupy a lot of space (like 10+Gb). And were way to long to create. So I had to prepare zip files before the first client requests an archive.

With this patch no additional files are created, zip file is streamed right away (and due to low compression rate there is almost no overhead in terms of CPU).
I also had to change fnNotFound() slightly to make it easier to guess what's wrong. It looks to be useful anyway.

I understand that the patch doesn't meet guidelines. As soon as you like the idea itself - I will tidy up things.
I also used code from https://github.com/maennchen/ZipStream-PHP (MIT)
and https://github.com/php-fig/http-message (MIT)

[electerious]

This PR will be closed because I'm not actively working on Lychee anymore. The repository of Lychee will stay available, but the main development continues under the Lychee Organisation. A fork maintained by the community.