Reduce Sentry transaction sample rate (#854)

## Status

- Related to
RaspberryPiFoundation/digital-editor-issues#1425

## Points for consideration:

- Performance - this should have a [performance
benefit](https://docs.sentry.io/platforms/javascript/sampling/#sampling-transaction-events)

## What's changed?

- Reduces Sentry transaction sample rate

Author: mwtrew

config/initializers/sentry.rb

@@ -5,5 +5,5 @@
   config.breadcrumbs_logger = [:active_support_logger]
   config.environment = ENV.fetch('SENTRY_CURRENT_ENV', nil) || ENV.fetch('RAILS_ENV', nil)
 
-  config.traces_sample_rate = 0.5
+  config.traces_sample_rate = 0.1
 end