Ensure there is a main item when the wiki page with the last main item is deleted (WIP)

TODO open a dialogue where the user can select a wiki page for main menu item respawn - see 'change parent wiki page'
Make wiki_menu_items#update more readable
Disable wiki module when last wiki page is destroyed
Redirect to projects#show after the last wiki page has been deleted
Destroy wiki after its last page has been deactivated and the wiki module has been successfully deactivated...


... This ensures a new menu item is spawned on reactivation of the wiki module. Moreover this is the raw state of a project without enabled wiki module.
Add action for main wiki menu item replacement
Deimplement automatic wiki main menu item restauration
Integrate main wiki menu item replacement selection into page flow (WIP)


TODO Add specs and happy path integration test
Remove duplicate initialization of @project in wiki menu items controller


... already done by before filter
Add controller specs for main wiki menu item replacement, fix action
Transfer menu item option to new wiki menu item during replacement
Fix wiki menu item replacement in the case no new main item is selected
Add integration test for main wiki menu item replacement
Merge permissions concerning wiki menu item management
Merge pull request #560 from opf/bugfix/2363_with_main_item_selection