renames link_to_issue to link_to_work_package
lets links on issue#index point to work_package#show
adapts path to work package in timelines
unifies activity across work packages

This also changes back acts_as_activity_provider from using joins to includes. Joins do not eager load which is required for avoiding requests on activity rendering
extends link_to_work_package to replace link_to_planning_element
moved, fixed and shortened  work_package link helper
updates changelog
Merge pull request #318 from opf/feature/cleanup_wp_links