Use a GDPR-compliant configuration for Ahoy

As mentioned in Ahoy's README [1]:

> Ahoy provides a number of options to help with GDPR compliance.
> Update config/initializers/ahoy.rb with:
>
> class Ahoy::Store < Ahoy::DatabaseStore
>   def authenticate(data)
>     # disables automatic linking of visits and users
>   end
> end
>
> Ahoy.mask_ips = true
> Ahoy.cookies = :none

As also mentioned in the README:

> If Ahoy was installed before v5, add an index before making this
> change.
> (...)
> For Active Record, create a migration with:
> add_index :ahoy_visits, [:visitor_token, :started_at]

However, the `visitor_token` doesn't exist in our table, since we
generated the `visits` table when Ahoy used the `visitor_id` column. So
we're using this column for the index.

Note we also need to change the `visit` method, since otherwise we get
an exception [2]. As mentioned on the issue reporting the exception:

> you'll need to copy the latest version of that method and adapt it to
> your model. I believe you'll want to replace:
>
> where(visit_token: ahoy.visit_token) with
> where(id: ensure_uuid(ahoy.visit_token))
>
> where(visitor_token: ahoy.visitor_token) with
> where(visitor_id: ensure_uuid(ahoy.visitor_token))

So we're copying the latest version of that method and changing it
accordingly.

[1] https://github.com/ankane/ahoy/blob/v5.0.2/README.md
[2] Issue 549 in https://github.com/ankane/ahoy

.rubocop.yml

@@ -396,6 +396,8 @@ Rails/FindBy:
   Enabled: true
   Include:
     - "**/*.rb"
+  Exclude:
+    - "config/initializers/ahoy.rb"
 
 Rails/FindEach:
   Enabled: true