To reproduce:
0. Start an odoo v10 instance with --load=saas_worker,web
1. Install ecommerce.
2. Enable "Allow external users to sign up" and "Enable password
reset from Login page" from General Settings.
3. Open different session then signup a new user.
4. After successfull signup, the new user will be redirected
to the backend (/web).

Facts to consider:
1. odoo.addons.auth_signup.controllers.main.AuthSignupHome and
odoo.addons.website.controllers.main.Website both inherit
odoo.addons.web.controllers.main.Home
2. When instantiating an odoo instance *without* saas_worker,web,
the mro is the following:

( <class 'odoo.http.Home (extended by Website, AuthSignupHome)'>
, <class 'odoo.addons.auth_signup.controllers.main.AuthSignupHome'>
, <class 'odoo.addons.website.controllers.main.Website'>
, <class 'odoo.addons.web.controllers.main.Home'>
, <class 'odoo.http.Controller'>
, <type 'object'>
)

while the mro *with* saas_worker,web loaded is:

( <class 'odoo.http.Home (extended by AuthSignupHome, Website)'>
, <class 'odoo.addons.website.controllers.main.Website'>
, <class 'odoo.addons.auth_signup.controllers.main.AuthSignupHome'>
, <class 'odoo.addons.web.controllers.main.Home'>
, <class 'odoo.http.Controller'>
, <type 'object'>
)

You can notice that depending on how the instance is instantiated,
the order of inheritance is different.

The problem occurs when saas_worker is loaded, so this bug can be
experienced by saas clients.

Explanation of the fix:
Notice that the original code calls web_login of its super in its
web_auth_signup method. This is technique is used normally during
optimization (according to RCO). If website is installed, the
portal user should be redirected to '/' instead of '/web' and this
is defined in web_login of website. However, the web_login of
"website" is not called after signup because "website" is not super
of "auth_signup" when saas_worker is loaded (see the mro above).

Calling self.web_login will make sure that web_login is called from
top to bottom, and regardless of the order of website and auth_signup,
web_login of "website" will be called and makes sure that the
new portal user is redirected to the '/' and not to '/web'.

opw-1956980

closes odoo/odoo#32741

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>