Fix/wp filter helper options

https://www.openproject.org/work_packages/7472
(see comment #36)

Options can be passed to the path helpers which is necessary for the atom links.

Updated the links on the roadmap view to use the new url helpers.

Wrote some specs.
_Author_ and _assignee_ are not properly passed to the work package view. The values for these filters are missing in the filter list.