Adds field to add projectless items to new project

The new project view includes a multi-select field listing all of the user's
items which are projectless and parentless. Selecting them sets the new
project's item_ids array.

Items will propogate their project_id to their children on save.
Adds item validation that project_id exists

Validation also accepts null.

Also, restricts new project form to only display parentless items to add.