`bbs_database add` does not put the abstract in the sentences table

[jankrepl]

🐛 Bug description

Currently, the add command does not put the abstract in the sentences table. One problematic implication of this behavior is that adding pubmed articles will always result in an error. Or in other words, we don't extract any text from pubmed articles.

https://github.com/BlueBrain/Search/blob/e2704e2413efddde016864612822d1f787e85dd4/src/bluesearch/entrypoint/database/add.py#L175-L176