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/controllers/application_controller.rb

@@ -3,6 +3,18 @@ class ApplicationController < ActionController::Base
     render :text => exception, :status => 500
   end
 
+  before_action :sentry_set_user
+
+  def sentry_set_user
+    return unless Setting.sentry_enabled
+
+    if user_signed_in?
+      Sentry.set_user(id: current_user.id, username: current_user.cn)
+    else
+      Sentry.set_user({})
+    end
+  end
+
   def require_user_signed_in
     unless user_signed_in?
       redirect_to welcome_path and return