Track ahoy visits on the server side

In commit 96ae69fe9, we stopped using cookies to track Ahoy visits and
started using a combination of the IP and the browser agent instead.

However, since we're still using the legacy logic from Ahoy 1.x to track
visits (which we had to add in commit b5220effd), this way of tracking
visits doesn't work and counts every page visited by a user as an
independent visit.

Maybe we could migrate existing data, which uses the `visitor_id` column
so it uses the new `visit_token` and `visitor_token` columns, but
there's no mention in the Ahoy documentation regarding how to do so.

While deciding what to do about this, we found something interesting.

For two years, we've been seeing random failures in the
`system/admin/tenants_spec.rb` tests, with messages like:

```
1) Tenants Create Tenant with subdomain
     Failure/Error:
       raise TenantNotFound, <<~EXCEPTION_MESSAGE
         Could not set search path to schemas, they may be invalid:
           "#{tenant}" #{full_search_path}.
         Original error: #{exception.class}: #{exception}
       EXCEPTION_MESSAGE

     Apartment::TenantNotFound:
       Could not set search path to schemas, they may be invalid:
         "earth" "public", "shared_extensions".
       Original error:
         ActiveRecord::StatementInvalid: Could not find schema earth
```

And we've found one of the causes: the AJAX requests done by Ahoy to
track visits. Sometimes a test that creates or updates a tenant finishes
but the Ahoy AJAX request to, say, `earth.lvh.me/ahoy/visits`, is
handled by the next test, when the `earth` schema no longer exists, thus
raising an `Apartment::TenantNotFound` exception.

So by disabling these AJAX requests and tracking the visits in the
server instead, we're killing two birds in one stone: we're fixing the
bug regarding the visits count and we're reducing the flakiness in our
test suite. It looks like we're also removing the "phantom ahoy cookie"
we were getting since the mentioned commit b5220effd: an ahoy cookie was
quickly set and unset in the browser.

Note that, even though we aren't migrating any data, we're still adding
the new fields, because some tests started to fail because, when
tracking visits in the server without cookies, Ahoy expects the Visit
model to have a `visit_token` field.

config/initializers/ahoy.rb

@@ -1,5 +1,3 @@
-Ahoy.api = true
-Ahoy.server_side_visits = :when_needed
 Ahoy.mask_ips = true
 Ahoy.cookies = :none