Loading views for custom models from module data files was not possible because
custom models and fields were introduced into the registry after all modules
were loaded. As a consequence, the view architecture did not pass the checks.
This patch takes a different approach: custom models and fields are loaded
early on in the registry, so that views can be validated. The trick is to take
special care of relational custom fields: we skip them if their comodel does
not appear in the registry. This allows to install and upgrade modules that
create/modify custom models, fields and views for them.
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