HTML would be stripped in `truncate_if_block` when a comment had multiple lines
and the first wasn't long enough to be truncated. The use of `node.content` would
strip all HTML tags. Using `node.inner_html` retains these tags and puts the
"..." in between paragraph tags.

Closes #2586

Closes #2586

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

Closes #1721

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

Removes header and table of contents processing from Redcarpet renderer.

Didn't like relying so much on Regex matches

Removes emoji-centric tests from GFM specs 

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

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

Using gitlab url to build links for gitlab issue tracker and add a spec. Fix rubocop warnings in path.

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.

Signed-off-by: Jeroen van Baarsen <jeroenvanbaarsen@gmail.com>