Handle file fields within transient models (use case: sale.config.settings)

[leemannd]

When exporting sale.config.setting we faced 2 issues:

• field sale_note should be moved to an html file
• special fields export is actually not working w/ transient models

You'll find WIP => here

[simahawk]

back on this: I'm not even sure if we should export such fields in any case. Example:

odoo/src/addons/sale/models/res_config_settings.py:12:    sale_note = fields.Text(related='company_id.sale_note', string="Terms & Conditions")

as you see that field is stored on the company itself which we should export anyway...
Relates to #101

[simahawk]

export settings deprecated