Add support for mentions references

[vkbo]

Summary:

This PR adds a @mention keyword that can be used to list any note tag that is mentioned in a piece of text, but not present. For instance, the characters in a scene talks about a character that isn't there, but you want to be able to track the mention so that you can later go back and check the character's consistency.

Related Issue(s):

Closes #1822

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