cubicweb #154090 should be warned when '?' or '1' cardinality implies a replacement of an existing relation [waiting feedback]

for instance, go to a blog entry affected to a blog, select a new blog for the entry_of relation. The existing relation will then be removed without any warning or question.

prioritynormal
typebug
done in<not specified>
load1.000
load left1.000
closed by<not specified>