Add a system administration page for disease groups

* Accessed from an Administration navbar menu that only appears if the logged-in user is an administrator
* Loads all disease groups upon page display, and currently that's it
* There are a couple of placeholder panels
I think I'll leave this for now - it would potentially affect the controller, kick-start JavaScript, Knockout JavaScript, Freemarker file, and corresponding tests. If it looks messy when we add multiple files (rather than starting them all with Admin in the current locations) then we can revisit this.