[IMP] website, website_partner: use fields from mixin on res.partner

Currently, res.partner has:
1. `website_id` field hardcoded in website module
2. `website_published` field from website `website.published.mixin` in
   website_partner module.

At the end, the res.partner model has all the field of the mixin
`website.published.multi.mixin` without having the mixin.
As a result, we have all the multi-website fields but we don't have access to
the mixins methods & computes.

Now, we use the `website.published.multi.mixin` on res.partner in website
module even if `website_published` is not needed if website_partner is not
installed.

+ add tooltip to `website` field to prevent warning since 2 fields would have
  the same label

closes odoo/odoo#29711

addons/website/models/res_partner.py

@@ -3,7 +3,7 @@
 
 import werkzeug
 
-from odoo import api, fields, models
+from odoo import api, models
 
 
 def urlplus(url, params):
@@ -11,9 +11,8 @@ def urlplus(url, params):
 
 
 class Partner(models.Model):
-    _inherit = "res.partner"
-
-    website_id = fields.Many2one('website', string='Registration Website')
+    _name = 'res.partner'
+    _inherit = ['res.partner', 'website.published.multi.mixin']
 
     @api.multi
     def google_map_img(self, zoom=8, width=298, height=298):

addons/website_partner/models/res_partner.py

@@ -6,7 +6,7 @@ from odoo.addons.http_routing.models.ir_http import slug
 
 class WebsiteResPartner(models.Model):
     _name = 'res.partner'
-    _inherit = ['res.partner', 'website.seo.metadata', 'website.published.mixin']
+    _inherit = ['res.partner', 'website.seo.metadata']
 
     website_description = fields.Html('Website Partner Full Description', strip_style=True)
     website_short_description = fields.Text('Website Partner Short Description')

odoo/addons/base/models/res_partner.py

@@ -163,7 +163,7 @@ class Partner(models.Model):
       help='The internal user in charge of this contact.')
     vat = fields.Char(string='Tax ID', help="The Tax Identification Number. Complete it if the contact is subjected to government taxes. Used in some legal statements.")
     bank_ids = fields.One2many('res.partner.bank', 'partner_id', string='Banks')
-    website = fields.Char()
+    website = fields.Char('Website Link')
     comment = fields.Text(string='Notes')
 
     category_id = fields.Many2many('res.partner.category', column1='partner_id',