[pkg] prepare 3.19.12

[web/views/formrenderer] remove `cubicweb:target` attribute on form once the target <iframe> is inserted For some unknown reason, IE9 does not want to submit the form inside the <iframe> when the ``cubicweb:target`` attribute exists. (closes #5377672)

[views] implement a custom json error view If the client asked for ``(e)jsonexport`` view, answer him with a json response rather than an error formatted in a big HTML blob. add a custom _requested_vid predicate. In cw 3.21, it will be deprecated in favor of ``match_form_params(vid=('jsonexport', 'ejsonexport'))``

don't override req.form['vid'] in the publisher's error_handler closes #5359234

when some inlined relation is set using cw_edited, its security shouldn't be checked. It's currently checked anyway because upon modification of `cw_edited`, `entity.skip_security` is updated to avoid security checking, but this is only considered for attributes, not relations. Closes #5477315

[test] stop depending on the email cube for hooks test Backport necessary part of its schema.

[schemas] cwuri should be read-only Closes #5457566

fix EditController._ordered_formparams method in case some mandatory relation isn't specified If you have an HTML form which doesn't carry information about some mandatory relation (maybe because this relation is set by a hook), you ends up with a KeyError when the form is posted. Using a get instead of subscription protocol fixes the problem. Closes #5481617.

[views] take into account subvid form param in SameETypeListView closes #5098473

[doc] fix errors in test_visibility_propagation example * forget to use the access instructions * wrong use of the function entity_from_eid * some indentation problems