Add (optional) Sentry integration

A Sentry DSN can be set via `SENTRY_DSN` and authenticated users will be
tagged with ID and username (cn) in events.

config/initializers/sentry.rb

@@ -0,0 +1,9 @@
+if ENV["SENTRY_DSN"].present?
+  Sentry.init do |config|
+    config.dsn = ENV["SENTRY_DSN"]
+    config.breadcrumbs_logger = [:active_support_logger, :http_logger]
+    config.traces_sampler = lambda do |context|
+      true
+    end
+  end
+end