[req, doc] add some documentation of CubicWebRequestBase attributes

[test] more standalone test add a __main__ section to `etwist/test/unittest_server.py`

[web templates] fix invalid html in main-no-top template; closes #2174806

[repository] implement a generic way to call repo-side services; closes #2203418

[web] Exposes the undo feature to user through a undo-history view (closes #893940)

[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