jobs: add ability to search jobs on /jobs page.

Add ability to search jobs on the /jobs page by job type or by status.

Fixes #2577 (Search filters for delayed jobs). This wasn't possible
before with DelayedJobs because it stored the job data in a YAML string,
which made it difficult to search jobs by type. GoodJobs stores job data
in a JSON object, which is easier to search in Postgres.

app/policies/background_job_policy.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+class BackgroundJobPolicy < ApplicationPolicy
+  def index?
+    true
+  end
+
+  def update?
+    user.is_admin?
+  end
+
+  alias_method :cancel?, :update?
+  alias_method :destroy?, :update?
+  alias_method :retry?, :update?
+  alias_method :run?, :update?
+end