[IMP] models: when checking for data in a table, do not use column 'id'

Replace the query "SELECT min(id) FROM xxx" by "SELECT 1 FROM xxx LIMIT 1".
Both requests are as efficient, and the second one does not crash if column
'id' is missing.
[FIX] website: prevent t-field on selection fields

This quick fix avoids a buggy behaviour in version 8.0 that could
confuse users.

A future version should implement properly selection fields in t-field.

(closes #2490)

(cherry picked from commit fe3cac30e4c5c132da1de02576d4aa325979ccd9)
[IMP] website_sale : avoid duplicated order lines

Add line id to while calling _cart_update() from sale_get_order()
The missing line_id parametre was making the _cart_find_product_line() call to fail as it was linked to an option while searching for lines without options (making the method recreate new lines).
Fixes #2573