Enable or Disable maintenance and registrations

This will allow any admin to enable or disable a pool maintenance mode.
Regular users that are still logged in will not be kicked out (yet). New
users that are trying to login and are not admins will be denied access.

Also added registration option to Admin Settings via WebUI. You can now
disable new accounts via web option or configuration setting.

Fixes #259 but also adds an additional feature.
Merge pull request #289 from TheSerapher/issue-259

Enable or Disable maintenance and registrations