[WebApp] Rectification related to plugins changes in WebApp 4.0

[marty]

Last month we announced upcoming changes in WebApp version 4.0 related to plugins.
Due to unresolved issues related to settings, we decided to scrap and revert these new plugin changes.

This means the plugins and default enabling plugins by the administrator will work the same way as it did in WebApp 3.5.x.
No action is required for the administrator or a plugin developer anymore.

A WebApp 4.0 (master) build with the revert will be pushed to the download server in the near future.