Enable compatibility with Asset Pipeline Plugin / Grails 2.4

The changes make it possible to use the Asset Pipeline Plugin instead of the Resources Plugin without breaking the design of the Platform Core views.
Backwards compatibility with the Resources Plugin is maintained by using `<plugin:isAvailable/>`.

`layouts/platform/dev.gsp` should work as is with the modifications. `layouts/platform/overlay.gsp` requires modifications in the plugins that use it, if those plugins want to be compatible with the Asset Pipeline Plugin (a pull request for the `platform-ui` will follow shortly).

Without modifications, the layout of those pages is broken (because of missing stylesheets), but nothing more. When using the Resource Plugin, though, everything continues to work as before.