Apply new item rules in project tree

[vkbo]

Summary:

This PR adds a small check when item selection in the project tree changes whether the selected item can have a document added to it. If it cannot, the corresponding options in the new items menu are hidden.

Related Issue(s):

Closes #1404

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed