Purpose
=======
Make placeholder look more like sample text to avoid users confusing it
with actual text, add "Sample" ribbon to default course context to avoid
confusing some users.
Specifications
=============
Change default placeholder text, add css rule for placeholders in
website_slide to make the colour lighter and disappear on focus.
PR: odoo/odoo/pull/78845
Task-2674361
closesodoo/odoo#78845
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>