If dpkg is active, show warning and blocked install/upgrade buttons

Created by: JoKeyser

Hi, this is an attempt at fixing issue #625 (closed) , including a screenshot. I'm unsure about the code, would be glad to receive comments. There was a lot of trial and error. For example, I'm somewhat aware of the messages, but they didn't seem appropriate since they show up everywhere(?) and are always closable(?). Any comments how to improve the logic very welcome, Django is completely new to me.