Patch H3 to support srcset attribute

There is an IA patch to enable harvesting of srcset with heritrix: https://github.com/internetarchive/heritrix3/pull/179/files

This is a major issue for netarkivet.dk affecting some of the largest and most important sites: https://sbprojects.statsbiblioteket.dk/pages/viewpage.action?pageId=31491867

(See also Netarkivet issue
https://sbprojects.statsbiblioteket.dk/jira/browse/NARK-1554 )

The task consists of patching our h3 with the relevant IA commit, test that it makes a difference to srcset harvest, then deploy the new patched h3 to nexus, and finally test that we can build and deploy NAS with the new patched h3.

Strategies for patching:

1. Create a patch in github and apply it to our code, or
2. Add IA heritrix as an upstream repository to a local git h3 checkout, then cherry pick the relevant commit.