[CWEP002 migration] support add_relation_type for computed relations Related to #3546717.

[CWEP002 migration] support drop_relation_type for computed relations Related to #3546717.

[CWEP002 migration] properly raise exception on (add|drop)_relation_definition for computed relation Related to #3546717

[hooks] do not abuse of inheritance for CWRType hooks Just copy the selector to ease readability.

[schema serial] add some comments

[schema] define full_rql on RQLExpression class without it, we may have an error while displaying error which occured during some expression initialization because this attribute is not yet defined while used in the __str__ method.

[schema] properly raise BadSchemaDefinition when some expression mains variables may not be guessed

[CWEP002] properly handle serialization of computed relations We now: * have CWComputedRelation in the bootstrap schema to store computed relations * properly serialize/deserialize it * test first if the database has been migrated and contains the related table Related to #3546717 [jcr: adjust unittest_querier to pass with the added entity type]

[CWEP002] Plug the computed relation rewriter in the querier Related to #3546717.

[CWEP002] refactor rql read security checking Split 'check_read_perms' into 'check_relations_perms' which checks relations 'read' permissions and 'get_local_checks' which build dictionary of local security checks (rql expression) for variables. This allows to check relations 'read' permissions earlier in the process and so to prepare insertion of the rql rewriter: we want to check permissions of the computed relation, not permissions of relations introduced by the associated rule, to conform to the CWEP. Related to #3546717