DEV: Remove HTML setting type and sanitization logic. (#14440)

* DEV: Remove HTML setting type and sanitization logic.

We concluded that we don't want settings to contain HTML, so I'm removing the setting type and sanitization logic. Additionally, we no longer allow the global-notice text to contain HTML.

I searched for usages of this setting type in the `all-the-plugins` repo and found none, so I haven't added a migration for existing settings.

* Mark Global notices containing links as HTML Safe.

db/migrate/20210928161912_migrate_deprecated_html_setting_type.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+class MigrateDeprecatedHtmlSettingType < ActiveRecord::Migration[6.1]
+  def up
+    execute <<~SQL
+      UPDATE site_settings
+      SET data_type = 1
+      WHERE data_type = 25
+    SQL
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end