Add footnotes

[vkbo]

Summary:

This PR adds support for footnotes. It uses a [footnote:key] shortcode where the footnote marker is added, and where key is a unique key. The footnote text is defined by a comment line on the format: %Footnote.key: Footnote text. The footnote text must be placed in the same document.

The footnote key is generated when a footnote is inserted from the menu. There are 810000 possible keys with the implemented algorithm, and it will look for collisions in active documents. However, documents in Trash and Archive are not indexed, so it is possible to have collisions. The unique keys between documents are mostly to help avoiding collisions when merging documents with footnotes.

Footnotes are currently exported to ODT, HTML and Markdown.

In addition, this PR:

• Allows formatting in comments, including regular comments. These were previously not allowed, but are needed in footnotes.
• Adds syntax highlighting for formatting in comments.

ToDo:

• Ensure that the footnotes are not polluted between documents, even if the key is unique.
• Make sure synopsis and short comments have the formatting stripped in the index.
• Look into adding a different highlight colour for comment text for special comments.
• Add test coverage of new code.

Related Issue(s):

Closes #342

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