fetch release note from github to keep in release_metadata table

In the web-app when browsing a release, the release message can be the release name and provide very little information, because the only message kept is the one stored in git. See: https://archive.softwareheritage.org/browse/origin/https://github.com/RaRe-Technologies/gensim/releases/

On Github the release page has a detailed description that should be kept with the release.

Example: https://github.com/RaRe-Technologies/gensim/releases/tag/3.4.0

"body" element in : https://api.github.com/repos/RaRe-Technologies/gensim/releases/tags/3.4.0

Two options:

1. have it directly in the release object
2. create a release_metadata table for all the stuff found with the release including assets or links to assets
3. put in ERMDS on the release SWHID

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Migrated from T1101 (view on Phabricator)