Move share column to base instead of module share.

The column is used to filter some groups and make them hidden from the res.users
form. However, if a module was loaded before the module share (e.g. depends only
from base), when upgraded, the eventual update of a res.group regenerated the
res.user view (to compute in_group_XX fields). This recomputation was done
before the module share was loaded in the registry (due to the dependency graph)
and the override of get_application_groups was not done, including shared groups
in the user form.

As we can not guarantee that a module is loaded, better to move to base module.
As share depends only of mail and is autoinstall, it was installed on most
instances anyway.

Fixes #6324
Fixes #5820