[#159] Use thumbnail for partner site map popup
[#159] Use thumbnail for partner site map popup

The map of projects on the partner sites home page uses the original
image in the popups dispayed when you click a pin.

Fix by using the named thumbnail "map_thumb" instead.

akvo/akvo-rsr,akvo/akvo-rsr,akvo/akvo-rsr,akvo/akvo-rsr