[FIX] http_routing, website: prevent crash when using `fw` in url

Before this commit, the routing map generated and used would be the one from
the website the request is performed, instead of the one from the `fw` website
ID which will be the one we redirect the user to.

This issue was introduced with the routing map by website, be8fc229 and is
restricted to a single case: a publisher using the website switcher, and it
won't happen on next page naviguation/refresh as the `fw` website id will be
the same as the current website's ID. Thus there won't be any routing map
mismatch.

Step to reproduce:
  - Create a page on website 2, set it as homepage
  - Naviguate to website 1 on '/' url
  - Naviguate to website 2 on '/' url
This will raise a werkzeug error about `EndPoint not iterable`.

----- Technical analysis ------

This is the current flow:
1. `_dispatch()` is setting `website_routing` to `get_current_website()` -> 2
2. `_dispatch()` is calling `_match()`
3. `_match()` is calling `routing_map()` with key = `website_routing`, which
   was set to 2 in step 1.
4. `routing_map()` is calling `_generate_routing_rules()` which generate the
   rules based on `website_routing`, which was set to 2 in step 1.
5. `_dispatch()` authenticate the user by calling `_authenticate()`
6. `_dispatch()` is calling `_add_dispatch_parameter()`, where URL param `fw`
   is forced in session, so `get_current_website()` now return the correct
   `website_id` -> 1

The issue: in order to handle the `fw` URL parameter (step 6.), we need to
check the rights to ensure we can allow the website switch.
To check rights, user need to be authenticated (step 5.), which is done after
generating the routing map (2. & 3. & 4.).
The routing map is generated based on the current website (step 1.)

Step 6 depends of steps 5 which depends of steps 2/3/4 which depend of step 1,
but step 1 should depend of step 6, which is an impossible cycle.

closes odoo/odoo#69416

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