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

It was redundant because `gfm` also took options.

The patches reduces pressure on GC and reduces markdown processing time

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.

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

This just extends the HTML::Pipeline::SanitizationFilter with our custom
whitelist.

Removes header and table of contents processing from Redcarpet renderer.

Removes emoji-centric tests from GFM specs 💥

Commit ranges, commits, external issues, issues, labels, merge requests,
snippets, users.

Extract and re-insert links after sanitizing user markup.

Parse GFM references - labels, issues, MRs, etc. - after calling the
HTML Pipeline `SanitizationFilter` so that we can use non-whitelisted
attributes like `style`.

Return a `SafeBuffer` instead of a `String` from the `#gfm_with_options`
method so that Rails doesn't escape our markup.

Also add `<span>` to the sanitization whitelist to avoid breaking syntax
highlighting in code blocks.

When nesting task list items, the parent item is wrapped in a `<p>` tag.
Update the task list parser to handle these paragraph wrappers.

Use the `SanitizationFilter` class from the html-pipeline gem for inline
HTML instead of calling the Rails `sanitize` method.

Fixes issues #880, #723, #1113: Markdown must be rendered to XHTML, not HTML, when generating summary content for Atom feeds. Otherwise, content-less tags like <img> and <hr>, generated when issue descriptions, merge request descriptions, comments, or commit messages use emoji, horizontal rules, or images, are not terminated and make the Atom XML invalid.

This is what’s actually allowed when creating a user or a project in gitlab.

Make the following changes to deal with new behavior in Rails 4.1.2:

* Use nested resources to avoid slashes in arguments to path helpers.

Resolves #1673.