[FIX] website_blog: fix link on the top banner of /blog

Before this commit, following these steps:
- Go to /blog
- Activate the option Customize > Top banner - Name / Latest Post
- Disable the option Customize > Full Width Cover

The URL to which we are redirected when we click on the blog of the
post presented at the top of the page leads to an error.

Indeed the URL of that link is something like "/blog?blog=blog.blog(2,)"
because of the template wrongly using the 'blog_url' QueryURL which is
defined in the case where we are rendering a blog page where no specific
blog is selected. We define that as `QueryURL('/blog', ['tag'], ...)`
but then parts of the template used it like this: `blog_url(blog=X)`
thus generating an URL like "/blog?blog=blog.blog(2,)". Adding "blog" to
the list of params would not be right as would create "/blog/blog/2"
which is still wrong as we want "/blog/2". And of course the "/blog"
prefix in the QueryURL definition is needed in case we only specify a
tag via `blog_url(tag=X)` (we expect /blog/tag/X). Patching QueryURL or
making blog_url a custom function instead of a QueryURL instance could
be a solution but it was judged not stable enough. We'll do that in
master. Here we only support "/blog?blog=blog.blog(2,)" URLs.

Note that many parts of those templates do not use the `blog_url`
function and simply build the URL by hand, which is why the bug only
occurred for a very specific set of blog options.

opw-2882492

X-original-commit: 82472731
Part-of: odoo/odoo#104254
Co-authored-by: Guillaume (gdi) <gdi@odoo.com>