[FIX] website_event_track: fix agenda scroll stuttering

On Firefox and Safari, horizontal scrolling on an agenda with many items
stuttered.
This was likely caused by triggering too many scroll events, without
throttling them.

Steps to reproduce:
- On Firefox or Safari, run with demo data
- Go to the "Design Fair Los Angeles" agenda (click on the event > Talks
> Agenda. Alternatively, enter the URL directly: `/event/ID/agenda`,
with the right event ID)
- Resize the page (or open the dev tools) so that there is a horizontal
scrollbar.
- Scroll horizontally with a trackpad or with shift + mouse wheel.
=> The agenda stutters/shakes: as you go to the right, it sometimes
slightly go back left, and vice-versa.
You might have to scroll from left to right and right to left a few
times to see the issue.

closes odoo/odoo#285732

X-original-commit: 69f9ca25ce2be7a591454b10e7a27eefc84a943e
Signed-off-by: Francois Georis (fge) <fge@odoo.com>
Signed-off-by: Robin Lejeune (role) <role@odoo.com>