Notify Bugsnag on deprecations in staging & production

The behaviour was set to `notify` already but we were missing a
notification handler. Now we'll get alerted about deprecations which
were not covered by specs.

config/initializers/deprecations.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+ActiveSupport::Notifications.subscribe(/deprecation/) do |_name, _start, _finish, _id, payload|
+  e = ActiveSupport::DeprecationException.new(payload[:message])
+  e.set_backtrace(payload[:callstack].map(&:to_s))
+
+  Bugsnag.notify(e) do |report|
+    report.severity = "warning"
+    report.add_tab(
+      :deprecation,
+      payload.except(:callstack),
+    )
+  end
+end