In SEDA 2.1, only one Content node is permitted per archive unit (2.0 allow
multiple ones). This allows to skip the SEDAContent entity in our model and to
attach all its children to its parent
(SEDASeqAltArchiveUnitArchiveUnitRefIdManagement).

This patch replaces occurences of SEDAContent/seda_content and should have test
to green but there is still some work to do on the UI side.

Related to #16684017