AdminHttpHandler should be moved to react platform

[gdaniel]

First it should be renamed (see #237).

Then the handler is just a front-end for testing the react component, it doesn't make sense to have it at the core level. Originally it was designed to test all kinds of ChatPlatforms, but it is not the case for now, so we should move it to the appropriate repository.

This may be an issue because the admin handler serves some files that are located in core/resources, so we want to be sure the paths are correctly updated.