[FIX] mail: `o_Activity_userAvatar` aspect-ratio

This commit fixes an issue causing the activity's avatar image to exceed
the available vertical space when in portrait mode.

The avatar's width/height are now fixed and its pic is set to cover the
available room only.

task-2795252

closes odoo/odoo#86583

X-original-commit: efbff4dae62c5c72541be96045cc96e3acf46628
Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>