Provide a way to change cache duration

**Is your feature request related to a problem? Please describe.**

ZeroNet only supports cache for around 20 minutes and it is not possible to configure this. However, some sites are updated more frequently and some are updated less frequently.

**Describe the solution you'd like**

It should be possible to change cache duration both by instance owner (with `--cache_for` argument) and by site owner (with `content.json`).

Command argument should be default cache duration or all sites, but `content.json` entry should overwrite it. Site owners should also be able to specify cache duration for specific files and specific content types.

This would enable to cache sites that don't update often for long time and sites that update quickly for short time.

Begin by locating the command-line handling for --cache_for and the content.json parsing path; the issue names no source files or tests. Done means an instance-wide default works, site settings override it, and sites can specify durations for individual files and content types, with tests covering those cases.

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