Refactor text formats modules

[vkbo]

Summary:

As the output formats code growing in scope, a refactoring is needed. This PR helps keeping the block and text formats apart from the classes by moving them to enums, rename some variables to be more consistent with current use, and reorganises some shared code.

Processing changes:

• Handling of justified text has been moved from the format classes to the Tokenizer, so that it is now applied consistently between formats.
• Handling of Comments, Tags and References are now all in the Tokenizer, outputting a regular text paragraph with all special formatting set.
• Theme colours are handled from one single settings class and applied to each formatted document. They should now all look like the preview.

Related Issue(s):

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