Disable edit/delete buttons when no nickname is selected.

Change-Id: I0dea51dc48784cf8c4f2cfc07a31955d12090a6c
Fixes-Issue: CLIENT-508
Reviewed-on: http://gerrit.dmdirc.com/3404
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Chris Smith <chris@dmdirc.com>