Integrate negotiate in swh.core

!30 (closed) introduced content negotiation for flask-based APIs using the negotiate python package.

https://github.com/nickstenning/negotiate

This later is very small with no activity for years, so we should just integrate the part of the code we use un swh.core instead of adding yet another (with no debian package) dependency... It looks to be under the MIT license, so it should be okay.

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Migrated from T1499 (view on Phabricator)