Add support for referencing tags in novel documents

[vkbo]

Summary:

This PR adds support for referencing tags in novel documents using the @story reference keyword. The tags were already collected by the indexer, so it was as easy as adding the keyword to the valid list, and exposing it on the Outline panel.

This PR also adds the @mention references to the Outline details panel, where they were missing.

Related Issue(s):

Closes #1990

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