[FIX] website: redirect to case insensitive URL if not exact match

Before this commit, if a link to a page was not correct because of a
case mismatch, it would simply land on a 404 page.
While it's correct, as URL are case sensitive, it leads to a few bad UX
flow at the admin/editor level:
- Create a link in your page (on a text or a button eg), type an URL
  which does not exists (to create it after) like /Page
- Click on the link/button you just made, you are redirected to /Page
  which display a 404 with the "Create page" option (correct)
- When you click on that button, it will actually create a page with
  /page URL, leading to a mismatch between the URL you created and the
  page URL.
  Your link/button will still lead to a 404 URL as it points to /Page.

Since it's just a fallback when an exact URL match is not found, it
should not break anything and should not have bad impact at any level
(seo/speed etc).
Indeed:
- It's done through a 302 redirect
- `_serve_page()` is already a fallback case, so it will only make
  the `website.redirect` and 404 cases a bit slower due to the extra
  search query.

The only possible scenario seems to be if the user (mind the uppercase):
- Created a /Page page
- Created a redirect from /page to /another-page

In this case, /page won't land on /another-page but on /Page.
This flow seems unlikely and is not actually wrong either way.
At least, it certainly is less important than ensuring a case
insensitive fallback.

Finally, note that another solution would have been to either:
- Force page URL to lower case.
  -> This is not stable friendly, people might be relying on this to
     create pages with different casing:
     `/Batman-VII-The-Dark-Knight-Whatevers`, while not recommended,
     doesn't sounds idiot.
     On top of not being stable friendly, we probably want to keep
     offering this possibility
- Redirect all URLs to lowercase endpoints.
  -> This is obviously not stable and not Odoo's jobs. It should be
     something decided by the sysadmin and done at nginx (etc) level.

task-3110294
opw-3104030

closes odoo/odoo#109812

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>