Add a new tags and references panel

[vkbo]

Summary:

This PR:

• Adds a new Backreferences panel below the document viewer, which uses a list instead of the text blob of titles like before.
• Each category of tags are also listed in separate tabs.

Related Issue(s):

Closes #925
Closes #998

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