Use new zim API from zimscraperlib

[rgaudin]

Obviously depends on the merging and release of https://github.com/openzim/python_scraperlib/pull/31

• Use the new make_zim_file and get rid of the ZimInfo
• [unrelated] Also move the encoding meta in templates as it's annoying (see commit)

[rgaudin]

OK, we'll wait for the download+headers thing to get merged and make a scraperlib release