applets/webbrowser: don't use fullRepresentation in panel (plasmashell freeze)

Web Browser's fullRepresentation in a panel hard-freezes Plasma in an infinite representation switching loop, similar to how the notes applet did before (bug 505132). Unfortunately, the same solution does not seem to work here, it freezes even with minimum layout sizes unset, and none of my attempts at finding a proper solution went anywhere.

Instead, force the compactRepresentation when used in a panel. This is clearly not the optimal solution, but better than shipping a hard-freeze trap that may leave users unable to use their computer if they try to get a web page as a sidebar (or just play around a bit with different settings).