Replace Pylons routes for build_nav_icon

[frafra]

Fix #476

I am not sure why admin and about require harvester. as prefix instead of the variable dataset_type, but it seems to work for me.

This PR does not replace all the Pylons routes: there are still a lot of deprecation warnings. This is just the bare minimum to make ckanext-harvest work with the current CKAN. I also prefer to send small fixes as they can be more easily reviewed instead of trying to replace all the routes at once.

[Zharktas]

This requires conditionals, as the extension still needs to work on older ckan versions. Thats why tests are failing.

[frafra]

Any suggestion/example on how to handle these conditionals properly?

[tino097]

@frafra maybe you can use the

 if h.ckan_version() > '2.9' 

in the templates to use the newer routes