[web] remove unused argument from treeview._init_headers

[doc/3.20] mention CWEP-002

[web] stop accepting the magic __message form parameter This has been deprecated for a while, and replaced by _cwmsgid, which doesn't allow arbitrary content.

[web/request] add security_enabled method Just forward to the underlying repo connection. This can be useful in the ORM when dealing with a "code-only" attribute, i.e. something that shouldn't be directly user-visible but must be accessible from a web request.

[dataimport] Fix use of _create_copyfrom_buffer() (related to #3845572) This is used indirectly by SQLGenObjectStore when running on a PostgreSQL database. Regression introduced by commit 70056633085c.

[c-c configure] make it possible to specify the section for sources configuration (closes #3477678)

[server] fix 'cnx' variable confusion in DBG_SQL exception case The rollback handling expects 'cnx' to be the cubicweb Connection, but the DBG_SQL block was replacing it with an sql connection, leading to lulz down the line. Also remove obsolete getattr (the sqlite wrapping is now done at the cnxset level, so cnx.cnxset.cnx should be the right thing already).

Provide sufficient context to check 'delete' permission in AjaxEditRelationCtxComponent Call rdef.check only when both fromeid and toeid are available. Though only call it once (for the first encountered related entity). Factorize a bit to keep handling of CSS/JS addition the same. Closes #3670209.

merge 3.19.6 into 3.20 branch

Added tag cubicweb-version-3.19.6, cubicweb-debian-version-3.19.6-1, cubicweb-centos-version-3.19.6-1 for changeset 934341b848a6