Improve performance of follow recommendation scheduler (#16159)

Express follow_recommendations in terms of account_summaries rather than
accounts, integrate filters that are unconditionally used, and materialize
the resulting view.

This should result in the bulk of the computation being performed only once
instead of **once per recommendation language**.

db/migrate/20210505174616_update_follow_recommendations_to_version_2.rb

@@ -0,0 +1,18 @@
+class UpdateFollowRecommendationsToVersion2 < ActiveRecord::Migration[6.1]
+  # We're switching from a normal to a materialized view so we need
+  # custom `up` and `down` paths.
+
+  def up
+    drop_view :follow_recommendations
+    create_view :follow_recommendations, version: 2, materialized: true
+
+    # To be able to refresh the view concurrently,
+    # at least one unique index is required
+    safety_assured { add_index :follow_recommendations, :account_id, unique: true }
+  end
+
+  def down
+    drop_view :follow_recommendations, materialized: true
+    create_view :follow_recommendations, version: 1
+  end
+end