[FIX] website_theme_install,*: redirect to frontend after theme install

*: theme_bootswatch, theme_default

Before this commit, the first time a website was loading a theme, it would
not be redirected to the frontend afterward (even if that theme was already
installed on another website).
Any other theme installation on that website would redirect to frontend as it
should.

Indeed, the first time it would use the `act_url` returned by `next()` which
would only be called during the first time installation on the website.

All the `ir.action.todo` related to themes can be removed anyway, as you can't
install directly a theme. It can only be installed through website settings >
load a theme.
In which case we will always want to redirect to frontend.

task-2070463

closes odoo/odoo#37306

Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>