jobs: migrate saved searches to ActiveJob.

* Fix tests to run the searches for real instead of mocking everything out.

* Fix SavedSearch.populate to only use the read only database in
  production because in breaks things in tests. Specifically:
  the posts get created in one db connection but searched for in
  another, but the second transaction doesn't see the uncommitted posts
  in the first transaction, so the search doesn't work.

app/jobs/populate_saved_search_job.rb

@@ -0,0 +1,7 @@
+class PopulateSavedSearchJob < ApplicationJob
+  queue_as :default
+
+  def perform(query)
+    SavedSearch.populate(query)
+  end
+end

app/models/saved_search.rb

@@ -32,7 +32,7 @@ class SavedSearch < ApplicationRecord
             post_ids.merge(sub_ids)
             update_count += 1
           else
-            SavedSearch.delay(queue: "default").populate(query)
+            PopulateSavedSearchJob.perform_later(query)
           end
         end
         post_ids.to_a.sort.last(QUERY_LIMIT)
@@ -97,7 +97,7 @@ class SavedSearch < ApplicationRecord
         CurrentUser.as_system do
           redis_key = "search:#{query}"
           return if redis.exists(redis_key)
-          post_ids = Post.tag_match(query, read_only: true).limit(QUERY_LIMIT).pluck(:id)
+          post_ids = Post.tag_match(query, read_only: Rails.env.production?).limit(QUERY_LIMIT).pluck(:id)
           redis.sadd(redis_key, post_ids)
           redis.expire(redis_key, REDIS_EXPIRY)
         end