Make Gitlab::Tracking.enabled? the SSoT of tracking being enabled

- Consolidates several uses of Gitlab::CurrentSettings.snowplow_enabled?
- Allows room for other tracking collectors in the future

app/helpers/tracking_helper.rb

@@ -17,6 +17,6 @@ module TrackingHelper
 
   def tracking_enabled?
     Rails.env.production? &&
-      ::Gitlab::CurrentSettings.snowplow_enabled?
+      ::Gitlab::Tracking.enabled?
   end
 end

app/views/layouts/_loading_hints.html.haml

@@ -7,5 +7,5 @@
   - else
     %link{ { rel: 'preload', href: stylesheet_url('application'), as: 'style' }, ActionController::Base.asset_host ? { crossorigin: 'anonymous' } : {} }
   %link{ { rel: 'preload', href: stylesheet_url("highlight/themes/#{user_color_scheme}"), as: 'style' }, ActionController::Base.asset_host ? { crossorigin: 'anonymous' } : {} }
-  - if Gitlab::CurrentSettings.snowplow_enabled? && Gitlab::CurrentSettings.snowplow_collector_hostname
+  - if Gitlab::Tracking.enabled? && Gitlab::CurrentSettings.snowplow_collector_hostname
     %link{ rel: 'preconnect', href: Gitlab::CurrentSettings.snowplow_collector_hostname, crossorigin: '' }

app/views/layouts/_snowplow.html.haml

-- return unless Gitlab::CurrentSettings.snowplow_enabled?
+- return unless Gitlab::Tracking.enabled?
 
 = javascript_tag do
   :plain

lib/gitlab/tracking/destinations/snowplow.rb

@@ -18,7 +18,7 @@ module Gitlab
         private
 
         def enabled?
-          Gitlab::CurrentSettings.snowplow_enabled?
+          Gitlab::Tracking.enabled?
         end
 
         def tracker