[req] New method: RequestSessionBase.find(). This method does what find_entities and find_one_entity did, except it returns the resultset itself. In addition, it accepts 'reverse_' arguments and check that the relations actually exists on the entity before executing the query. Also, reimplement find_one_entity and find_entity based on the new function so they benefit from the more complete implementation, and deprecate them. Note: List of values in kwargs are NOT supported in this initial implementation. Closes #3361290

What's new in CubicWeb 4.0?
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Deprecated Code Drops
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* The ldapuser source has been dropped. ldapfeed is the only official source
  remaining for ldap.