Fix invalid Atom feeds when using emoji, horizontal rules, or images

This is a fix for 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* are not terminated and make the Atom XML invalid. Such tags are generated when issue descriptions, merge request descriptions, comments, or commit messages use emoji, horizontal rules, or images.

To pass this option through from the relevant Haml templates to the proper place in the `gfm()` method, a new method `gfm_with_options()` is introduced. It reuses the options dictionary passed to `markdown()` and interprets options `xhtml` and `parse_tasks` from it (the latter was a convenient replacement for `gfm_with_tasks()`). `xhtml` is already interpreted by Redcarpet::Render::HTML, but that alone was not sufficient, because the post-processing in `gfm()` would convert its XHTML tags back to HTML.

I found no way of passing additional optional options to the existing `gfm()` method without requiring updates to existing callers and without getting in the way of the existing optional arguments, but maybe someone who knows more about Ruby than I can think of one.

Thorough review appreciated since this is the first time I have used Ruby.

See merge request !344

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.

Allow smb:// links in Markdown text.

As requested by Sam McLeod at https://gitlab.com/gitlab-org/gitlab-ce/issues/1184

See merge request !1669

Also preserve code preview color scheme in events dashboard.

Assign default colors to all code blocks shown as <pre class="code highlight [color_scheme]">

Closes #1139

I decided to create a fork of rouge as rouge lacks a HTML formatter with
the required options such as wrapping a line with <span> tags.
Furthermore I was not really convinced about the clarity of rouge's
source code.

Rugments 1.0.0beta3 for now only includes some basic linting and a new
HTML formatter. Everything else should behave the same.

Make the Markdown parser recognize "[x]" or "[ ]" at the beginning of a
list item and turn it into a checkbox input.  Users who can modify the
issue or MR can toggle the checkboxes directly or edit the Markdown to
manage the tasks.  Task status is also displayed in the MR and issue
lists.