Change parsing of tasks to markdown description

[mcsken]

Background

Currently, we parse the description in HTML for open tasks. Because GitHub can change the relevant HTML classes at any time, this is fragile. Instead, we should use the Markdown description, where the syntax for tasks should be more stable as changes have an immediate impact on all GitHub users writing description.

Also, parsing the Markdown should be more energy efficient.

Goals

• The parser for open tasks uses the Markdown description instead of the HTML description.
• All TODO#29 comments are addressed.

Estimate: ?h