[FIX] mail: allow to create a manual blacklist model
Issue 1: before this patch it was impossible to create a manual model
marked as "Is blacklist". The reason is that a blacklist model
implicitly need an `email` field, but such field is impossible to add in
a manual model: the field must start with `x_`.
Solution: append `x_` to the implicit email field. Note, in principle
the user gets an error if `x_email` is not present. Solved by adding the
field when creating the custom model. Ideally we should show some hint
in the interface to make it more user friendly. That is out of the scope
of this patch.
Issue 2: when we have a manual model that is mail blacklist it's
impossible to create its model class. We get an error because the MRO is
not correct. The reason is that we are adding `mail.thread.blacklist`
_after_ `mail.thread` in the `_inherit` list. That list is used to
generated the `__bases__` of the model class[1]. According to Python's
MRO rules[2], since `mail.thread.blacklist` appears after `mail.thread`
as parents of the custom model class this order _must_ be respected. But
`mail.thread` must appear _before_ `mail.thread.blacklist` because the
latter inherits from the former. This is a contradiction and the MRO
algorithm cannot succeed. To put it in a simple example:
```py
class A: pass
class B(A): pass
# This fails:
# class C(A, B): pass
# The right order is:
class C(B, A): pass
# Equivalent to:
class D(B): pass
# C and D have the same MRO linearization excluding themselves
assert D.mro()[1:] == C.mro()[1:]
```
Example traceback:
```
Traceback (most recent call last):
File "/home/odoo/src/odoo/14.0/odoo/service/server.py", line 1201, in preload_registries
registry = Registry.new(dbname, update_module=update_module)
File "/home/odoo/src/odoo/14.0/odoo/modules/registry.py", line 89, in new
odoo.modules.load_modules(registry._db, force_demo, status, update_module)
File "/home/odoo/src/odoo/14.0/odoo/modules/loading.py", line 464, in load_modules
registry.setup_models(cr)
File "/home/odoo/src/odoo/14.0/odoo/modules/registry.py", line 263, in setup_models
env['ir.model']._add_manual_models()
File "/home/odoo/src/odoo/14.0/odoo/addons/base/models/ir_model.py", line 430, in _add_manual_models
Model = model_class._build_model(self.pool, cr)
File "/home/odoo/src/odoo/14.0/odoo/models.py", line 585, in _build_model
ModelClass.__bases__ = tuple(bases)
TypeError: Cannot create a consistent method resolution
order (MRO) for bases BaseModel, mail.thread, mail.thread.blacklist, base
```
Solution: check if a model inherits from `mail.thread.blacklist` first.
There is no need to add `mail.thread` if inheriting
`mail.thread.blacklist` because the inheritance is already implicit.
This issue was observed during upgrades. We convert custom models and
fields into manual to allow upgrading without custom code. This causes
issues because the MRO error appears when a custom model inherits mail
blacklist.
[1]: https://github.com/odoo/odoo/blob/02f820fb0eaddbb3a4269a0967184c8aaf52c363/odoo/models.py#L585
[2]: https://www.python.org/download/releases/2.3/mro/
closes odoo/odoo#119169
Signed-off-by: 
Christophe Simonis <chs@odoo.com>