merge stable heads

fix ticket #1382716 (problem was actually more subtle than I originally thought) * log errors happening in looping tasks and repo threads * do not swallow RegistryNotFound exceptions which occur during hook execution (only those happening during hook registry selection)

[web/views/basetemplates] nbsp breaks tests, use #160 instead

[web/views/workflow] hide tr_count attribute in wf transition form

backport stable

[html generation] even when doctype without extension are asked, we should define the xmlns:cubicweb attribute to avoid html validation error during tests (break tests with new calendar view in default branch since it demote to html)

handle the fact that there might be no views registered in some configurations (#1382716)

[ms] fix ms planner crash on some ambiguous queries, as demonstrated in a comment on http://www.cubicweb.org/ticket/1382452

[ms] #1382452: incorrect results with multi-sources when some relation is crossed among sources with non-invariant variables, we've to introduce an intermediary steps to fetch all relations from all sources, then make the join with intermediary tables containing possible variables values.

#1382705 forbid with sql level locking the simultaneous fireing of a transition on a given entity