Feature/rails3 activity module

integrate the activity_module plugin in the core

added some additional features/bugfixes:

* the overall activity page only shows activity from projects, where the module is activated
* links to activity are not rendered, if the module is disabled
* links to activity page ("1 hour ago" in issues#show for example) do not point to the projects activity page (not to the overall activity page anymore)

see: https://www.openproject.org/issues/1461