[FIX] website_blog: fix next post loading animation

Previously, when clicking on the next post at the bottom of a blog post,
a spinning loader circle would always appear in the bottom left of the
screen in an incorrect position. This commit removes that spinner.

This commit also adds support for the half-screen cover size, which
until now would animate as though it was full screen. All in all this
just makes the transition animation much more seamless, which was the
point of having it in the first place.

task-2166790

closes odoo/odoo#44198

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>