Remove the RandomData source

Problem/Motivation

I'm not sure why this source exists. It was probably useful aeons ago, but now that ProjectBrowserTestMock exists, and uses a hard-coded set of predictable projects and categories, what's the benefit of relying on random data for some tests? All it does, as far as I can tell, is add complexity and make the tests less predictable and harder to understand.

Proposed resolution

Remove the RandomData plugin, and replace all uses of it with ProjectBrowserTestMock instead.

Since project_browser_devel only exists to provide the random_data plugin, this will also render project_browser_devel obsolete. It will remain around for backwards compatibility reasons, but hollowed and explicitly marked obsolete. Anybody using it (and why the hell would you??) should uninstall it.