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.

app/models/setting.rb

@@ -10,6 +10,13 @@ class Setting < RailsSettings::Base
     account accounts donations mail webmaster support
   ]
 
+  #
+  # Sentry
+  #
+
+  field :sentry_enabled, type: :boolean, readonly: true,
+    default: (ENV["SENTRY_DSN"].present?.to_s || false)
+
   #
   # Discourse
   #