Always save pinned tabs in session

When policy is set the NEVER we obviously never write a session file. With
the pinned tabs option this does not seems to be a good option, so let's mimic
other browsers and store them even if policy is set to NEVER.

Fixes: https://gitlab.gnome.org/GNOME/epiphany/-/issues/1508
Part-of: <https://gitlab.gnome.org/GNOME/epiphany/-/merge_requests/997>