Allow admin_setting_confightmleditor to accept normal files

This lack of support for files inside admin settings seems to be a recurring common question.

It should be possible to make an admin setting accept files, and then some sort of small amount of glue or a naming convention or similar to it's easy to write the plugin file function to server them.

Most of the core htmleditors admin settings including 'summary','auth_instructions', 'maintenance_message' et would all benefit from this, but mostly about allowing easy support for this in plugins.