Extends API V2 PE controller
Removes PE controller and related specs
Deactivates AJAX logout test

This tests checks a confirmation modal. But modals are deactivated, so
should this test.
Removes test accessing the PE controller
Removes PE menu entry
Moves 'for_project' scope and removes unused scope
Adds changelog entry

[skip ci]
Removes planning element controller views

[skip ci]
Merge branch 'feature/rails3' into feature/rails3_move_pe_controller_to_api_v2

Conflicts:
	doc/CHANGELOG.md

[skip ci]
Merge branch 'feature/rails3' into feature/rails3_move_pe_controller_to_api_v2

Conflicts:
	app/controllers/planning_elements_controller.rb
	app/models/planning_element.rb
	app/views/planning_elements/_form.html.erb
	app/views/planning_elements/_planning_element.html.erb
	doc/CHANGELOG.md
Merge pull request #367 from opf/feature/rails3_move_pe_controller_to_api_v2

[FEATURE] Move PE controller to API V2