Add min-height for #cp-main .panel

If the ucp-main has not that much content it's shorter than the ucp-menu.

To make all look pretty and nice #cp-main .panel should get a min-height via css to cover the red area (attachment) with the panel background

My short test showed that it should be at least 175px, but I'm not sure if it's ok for every resolution. If anybody can tell me what value is ok, then I will submit a patch