Otherwise recomputation may fail with "record does not exist or has been
deleted" when creating records with a value directly set for e.g.
'image_mediun' on a new product.

The issue comes from the storage of the images.  When creating a product, the
creation of an attachment for storing the field `image_medium` triggers the
recomputation of that field before its dependency `image` is set.  As `image`
is initially null, `image_medium` is recomputed as null, and this deletes the
attachment just created before the latter has completed its creation!  As a
consequence, some code at the end of `create` for the attachment crashes
because the record has been deleted.

The following models have been fixed: `fleet.vehicle.model.brand`,
`hr.employee`, `im_livechat.channel`, `mail.channel`, `payment.acquirer`,
`pos.category`, `product.template`, `product.public.category`, `res.partner`.

opw 666330
Closes #11516

Do not compute the partner default image when being in testing or install mode.
For example when importing a huge list of contacts there is no need to compute
a default image for every contact.

Moreover the random colorize is more limited. There is no need to explore the
whole space of rgb. It is now limited by steps in the rgb random, leading to
less different images being possible. There are currently 512 possible
combinaisons.

Fixes #2569
Closes #6260

Fixes #2569
Closes #6260

- Preserved explicit 3rd-party copyright notices
- Explicit boilerplate should not be necessary - copyright law applies
  automatically in all countries thanks to Berne Convention + WTO rules,
  and a reference to the applicable license is clear enough.