finish yesterday work on rql constraints:
* Fix inheritance pb: there are some places where we filter constraints
according to the class hierarchy, so rql unique constraint should'nt
be neither a RQLContraint nor a RQLVocabularyConstraint subclass.
Added test in unittest_schema reflecting this.
* So now we have to get explicitly RQLUniqueConstraint where desired (eg in
server/hooks.py)
* Update i18ncubicweb command to include constraint types in generated pot file
(this should have been there for a while...)
* Update unittest_schemaserial which has been broken when serializing format
for rql constraints has changed
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