[MERGE] *: rename image fields and use image.mixin

Merge commit for PR #34925
Related enterprise PR odoo/enterprise/pull/4865

The goal is to:
- Use `image.mixin` on the models currently storing multiple image sizes.
- Create Image field and use it on models where mixin is overkill.
- Rename the image fields to contain the pixel number in the name.
- Add 512 size.
- Clean up `image.mixin`, zoom field, variant fields.

Note: models that currently store only a single size image will not be changed
in this PR.

Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>