FEATURE: Add IgnoredUsersSummary daily job (#7144)

* FEATURE: Add `IgnoredUsersSummary` daily job

## Why?

This is part of the [Ability to ignore a user feature](https://meta.discourse.org/t/ability-to-ignore-a-user/110254/8).

We want to:

1. Send an automatic group PM that goes out to moderators
2. When {x} users have Ignored the same user, threshold defined by a site setting, default of 5
3. Only send this message every X days which is defined by another site setting

db/migrate/20190314144755_add_summarized_at_column_to_ignored_users_table.rb

@@ -0,0 +1,5 @@
+class AddSummarizedAtColumnToIgnoredUsersTable < ActiveRecord::Migration[5.2]
+  def change
+    add_column :ignored_users, :summarized_at, :datetime
+  end
+end