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Closes #2433

Now, a reference link with a `.gfm` class but without one of our
`data-*-id` attributes should be shown to the user rather than hidden.

This module is now the sole source of knowledge for *how* we render
Markdown (and GFM).

This is to let us redact them later in another filter.

When this option is `:description`, we use a more restrictive whitelist.
This is used for Project and Group description fields.

Forces a `rel="nofollow"` attribute on all external links.

Fixes #1690

Fixes #1651

Instead of using a fork, we subclass the filter and only apply the
`task-list` class to list items that actually are task lists.

Closes #1645

See https://github.com/github/task_list/pull/60