[migration] Fix addition of entity type including boundary constraints on its own attributes This was failing because of the sequence of sql executing when adding an attribute: 1. add entity type <commit> 2. add relation type for attribute 1 <commit> 3. add attribute 1 and associated constraints <commit> etc. In the case of e.g. start/end constraint, we were trying to add the constraint before addition of the constrained attribute (e.g. add constraint on 'start' referencing 'end', but only 'start' has been added yet, not 'end'). This patch fix this by: * adding the relation type to the schema without having to commit, but keeping the operation to revert the addition if necessary - this allows to a single commit for all attributes of the entity type ; * using a LateOperation on constraint operation, so we ensure attributes are actually added before any constraint is added.

[schema sync] Refactor AfterAddCWRTypeHook and AfterAddCWComputedRTypeHook so the latter inherit from the former and enhance their docstring.

Drop embed-allowed option, gone away for a long time