[FIX] web: quick create inputs don't take all space

Adds `width: 100%` on the `.o_field_widget` so the input it can take the
all width in the "Quick Create" form.

Also, this commit fixes the CSS selector for the `margin`
rule as in the "Quick Create" form view there is no `.o_group` anymore
on `.o_inner_group`.

Steps to reproduce:
* Go to project
* Select a project
* Click on the `Create` button => BUG

closes odoo/odoo#101913

Signed-off-by: Adrien Dieudonné (adr) <adr@odoo.com>