Make add handle duplicate articles

[EmilieDel]

Fixes #576.

Description

bbs_database add is now filtering and keeping the articles not already present in the database (thanks to uid) before adding them.

Discussion

• If there are no new articles (=articles whose uid does not exist in the database), should we
  • raise an error
  • or log the fact that we did not add any articles because all of them are already present in the database and return 0 ?
• Currently, the database built in the conftest.py is inspired by the covid-19 database schema. Should we create a new database representing our future database ?
• test_sql contains a test checking that we do not have any SQL query outside of the sql module. We never updated this test after adding new modules. Should we do something about this ?
• Should we save more often the articles ? Currently we parse everything and then save all articles. Should we create batch ? Save articles one by one ?

Checklist

• This PR refers to an issue from the issue tracker.
  (if it is not the case, please create an issue first).
• Unit tests added.
  (if needed)
• Documentation and whatsnew.rst updated.
  (if needed)
• setup.py and requirements.txt updated with new dependencies.
  (if needed)
• Type annotations added.
  (if a function is added or modified)
• All CI tests pass.