DEV: Cleanup ignored user logic (#11107)

- IgnoredUser records should all now have an expiring_at value. This commit enforces that in the DB, and fixes any corrupt rows
- Changes to the ignored user list are now handled by the `/u/{username}/notification_level` endpoint. This allows setting expiration dates on the ignore. This commit removes the old logic for saving a list of usernames in the user preferences.
- Many specs were calling `IgnoredUser.create`. This commit changes them to use `Fabricate(:ignored_user)` for consistency

db/migrate/20201103103401_add_not_null_to_ignored_users.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+class AddNotNullToIgnoredUsers < ActiveRecord::Migration[6.0]
+  def up
+    execute <<~SQL
+      UPDATE ignored_users
+      SET expiring_at = created_at + interval '4 months'
+      WHERE expiring_at IS NULL
+    SQL
+
+    change_column_null :ignored_users, :expiring_at, false
+  end
+
+  def down
+    change_column_null :ignored_users, :expiring_at, true
+  end
+end