Improve Subversion loader and develop CVS loader

The proposal got approved and Stefan Sperling is going to work on this within the 2021 year.

This is the main task tracking the design, development, deployment and communication work efforts.

Communication related, here is [[ https://pad.inria.fr/p/np_wJ8pK0htMsgw7isP_svncvs | draft text for the announcement link ]].

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Migrated from T2845 (view on Phabricator)