[IMP] website: filter out unpublished records for non internal users

Issue:
Search on website has a big performance issue because the database has
too many product_template.

Analyze :
The website search mechanism has hardcoded sql query. And the generic
query don't have any filtering. Which lead to make a expensive search
(similarity) on the whole product_template even if the products are
not published and then displayed to the user.

Fix :
Filter the is_published field in the generic website method when the
model has a 'is_published' field and the request is not done by a user
(thus for customers or portal users).

Note:
The internal user is still able to search on unpublished product.Thus
have no performance improvement.

Benchmark:
| SQL Query | # Input data | Before PR | After PR |
|:---------:|:------------:|:---------:|:--------:|
|General best_similarity |700000 products (161 published) | 12.65 s | 0.12 s |
|FROM ir_translation|2,796,000 ir_translation | 6.338 s (586k hit) | 0.091 s (185 hit) |

Related task:
task-3473786

closes odoo/odoo#130734

Signed-off-by: Romain Derie (rde) <rde@odoo.com>