[FIX] website_forum: force image size in css to handle gifs

Issue

	- Preferences > Change your avatar with a big .gif
	- Install Forum
	- Go on a question

	The gif is not resized in the nav and in the post
	answer box

Cause

	In fact, all gifs are not really resized. They just
	look fine because there is CSS to handle the case
	where the server resize does not work.

Solution

	Do as it is done everywhere else, add CSS rules

OPW-2223260

closes odoo/odoo#48390

Signed-off-by: Jason Van Malder (jvm) <jvm@odoo.com>