config: disable Sentry sampling when not enabled

This removes spam in the debug console when running locally.

Removed messages look like a swarm of:

> [Tracing] Discarding <rails.request> transaction </assets/marianne.png> because it's not included in the random sample (sampling rate = 0.001)

config/initializers/sentry.rb

@@ -3,5 +3,5 @@ Sentry.init do |config|
   config.send_default_pii = false
   config.enabled_environments = ['production']
   config.breadcrumbs_logger = [:active_support_logger]
-  config.traces_sample_rate = 0.001
+  config.traces_sample_rate = ENV['SENTRY_ENABLED'] == 'enabled' ? 0.001 : nil
 end