Ensure HTML tags are properly closed in output documents

[vkbo]

Summary:

This PR rewrites the text formatting method of the HTML converter to ensure format tags aren't opened if they are already open in a paragraph, and always closed at the end of the paragraph. Since shortcodes don't actually ensure tags are closed, unlike the Markdown tag parsing, this is necessary to prevent generating unwanted formatting in the HTML.

Related Issue(s):

Closes #1919

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