Add convert to work item button to description markup

Proposal

• We need to add a <task-button></task-button> before every task list item. We can't do this conditionally by checking a feature flag, so the tag should be added every time. The tag will be invisible in a browser but the frontend will use it for the work items feature.

Considerations

• We might need to bump the CACHE_COMMONMARK_VERSION so the new tag exists in every issue description.