[undo, transactions] Adds a property in the transaction API. (closes #2202953) Required by forthcoming changes in undo_web.diff

[repo, undo] Finish repository-side implementation of the undo feature (closes #893940) 1) Changes API for undo_transaction in order to raise an exception: the previous API returned a sequence of error, the user of the API needed to be careful to check explicitly. We change this for it to raise an UndoException with the list of errors as an attribute. 2) The server-side support for undoing update actions is finished 3) The configuration for `undo-support` is now a simple boolean Beware of the costs, both in DB space and in number of requests on the RDBMS backend, once this feture is activated. 4) Adds a txuuid param/attribute to the existing NoSuchTransaction exception.

[rql rewrite] when a subquery has to be introduced, properly return the snippet expression so that further expressions are properly ored. Closes #2207180

[migration/3.14.4] use the right API else we break sqlserver instances (closes #2211248)

backport stable

[devtools] make a copy of the xvfb-run script so we can kill it properly Since changeset a3d3bdd46463 we don't run firefox directly but through a wrapper script. This meant that on stopping the test, we would kill that wrapper script but leave the Xvfb server and firefox processes running. So make a local copy of xvfb-run, and make it kill Xvfb when it goes down, not just when its client exits.

[ajax] use AJAX_BASE_URL instead of JSON_BASE_URL JSON_BASE_URL / 'json' should be replaced by AJAX_BASE_URL / 'ajax'

[web/config] refactor initialization of the data url Common logic have been moved in a dedicated `data_relpath` function.

backport stable

[test] create temporary directory with meaningfull prefix It will help cleanup temporary directory when cubicweb test failed to cleanup themself.

[schema] make cubicweb 3.14 compatible with yams 0.35

[test] fix ReST extensions tests