[dataimport] introduce the importer and extentity classes This introduces the ``ExtEntity`` class which is a transitional state between data at external source and the actual CubicWeb entities. ``ExtEntitiesImporter`` is then in charge to turn a bunch of ext entities into CW entities in repository, using a given store. This changeset also introduces ``SimpleImportLog`` and ``HTMLImportLog`` which implement the CW DataImportLog interface in order to show log messages in UI using simple text and HTML formats respectively, instead of storing these messages in database. Both have mostly been backported from cubes.skos.dataimport. Closes #5414753.

[test] Use store.prepare_insert_relation instead of deprecated relate method Follow-up for 1f5026e7d848. Related to #5040344.

[dataimport] since we have foreign key constraints in the database, 'entities' table should be updated first

[dataimport] Move stores to new API. Here is the final store API: * ``prepare_insert_entity(<entity type>, **kwargs) -> eid``: given an entity type, attributes and inlined relations, return the eid of the entity to be inserted, *with no guarantee that anything has been inserted in database*, * ``prepare_update_entity(<entity type>, eid, **kwargs) -> None``: given an entity type and eid, promise for update given attributes and inlined relations *with no guarantee that anything has been inserted in database*, * ``prepare_insert_relation(eid_from, rtype, eid_to) -> None``: indicate that a relation ``rtype`` should be added between entities with eids ``eid_from`` and ``eid_to``. Similarly to ``prepare_insert_entity()``, *there is no guarantee that the relation will be inserted in database*, * ``flush() -> None``: flush any temporary data to database. May be called several times during an import, * ``finish() -> None``: additional stuff to do after import is terminated. **Warning:** ``prepare_update_entity()`` still needs to be implemented for NoHookRQLObjectStore. Related to #5040344

[js] Using appropriate http method when calling a remote function to respect REST principles. closes #5355952

[server/test] Add a test for db-statement-timeout option Sadly I don't think there's a way to test that functionality without adding sleeps which make the test suite longer :/ Related to #2547026

[datafeed] give an error message if a source is missing a parser id Closes #3484231

[storage] use mkstemp to create files in bfss Avoids race condition, at the cost of slightly less predictable file names.