[IMP] website_slides: rename string for quiz attempts

When exporting fields, the reward attempts fields were split from each other. By grouping them, it allows
easier management from user as all rewards fields are now grouped in the export wizard.
Additionaly, they are also grouped in the dropdown for custom filter.

task-2155189

closes odoo/odoo#42103

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>