[FIX] mail_group: use default picture disc if not mimetype has been defined (else tb)
[FIX] website_sale: browse category to slugify. Because slug of an id will generate a tb 'need more than 1 value to unpack'
[FIX] account_voucher: getting default invoice crashes

Fixes #3524.
[FIX] ir_model: always setup models before calling _auto_init()
[IMP] models: in _init_manual_fields(), create fields instead of columns

Note that serialized fields are ignored; they are no longer supported, anyway.
[FIX] models: simplify conditions that seem to lead to missing fields
[IMP] models: improve performance of _setup_fields()

There was an issue in _setup_fields(): the method invokes _inherits_reload(),
which recomputes inherited fields, and invokes itself recursively on children
models.  This may be problematic if the children models have already been set
up.

This optimization avoids recursive calls of method _inherits_reload().  In
_setup_fields(), first all parent models are set up, then their fields are
inspected to determine inherited fields, and their setup is done.  This scheme
guarantees that inherited fields are computed once per model.
[FIX] fleet: avoid function field computation order issues

The name of a vehicle is function field.
The field odometer on a vehicle has a a setter that creates an odometer.
The name of an odometer is a function field that uses the name of the vehicle.
If a vehicle is created with a value in odometer field, the odometer is created before the name is computed so the concatanation would fail.
Fixes #3468
[FIX] point_of_sale: rounding error in discount computing + removing duplicated code