Vol ops fix

On production, the organisations with more than one vol op get a vol op popup that obscures the marker because the vol op info box is much larger than one with just a single vol op.

To avoid having to dynamically measure the length of the rendered infobox, we chose to, for the moment at least, make the infobox come out of the bottom of the icon which makes the pixel offset logic simpler.

We've also refactored out the pixel offset and other assorted marker type specific logic into a module that we can test because switching on the marker type offset everywhere just screamed for refactoring.

https://small-icons.herokuapp.com/ Here it is staged.