[FIX] base: allow user to modify its own preferences

The res.users model functions a bit uniquely since it should allow a user to
modify their own parameters but turns off model edition priviledges by default.
See "SELF_WRITEABLE_FIELDS".

This implies that ir_ui_view#_postprocess_access_rights method will by default
turn off record edition, by automatically adding a 'edit="false"' attribute on
the form node when the frontend calls 'get_views'.

This will in turn prevent the user from modifying its preferences as it will
set the form view in readonly mode.
(It was apparently ignored pre-OWL, hence why we only have this issue now).

To fix the issue, we force the edition by manually setting 'edit="1"' on our
"view_users_form_simple_modif" form.
Allowing the end-user to modify their own settings again.

A tour was added to ensure this behavior.
Note that tour steps need to be adapted in the 'hr' module, as this module
changes the flow of modifying user preferences.

Task-3067001

closes odoo/odoo#106125

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>