Merge branch 'rs-more-pipeline-filters' into 'master'

More HTML::Pipeline filters

The big part of this MR is a feature that is intended to test the entire Markdown-parsing process from beginning to end. See `spec/support/markdown_feature.rb` and `spec/features/markdown_spec.rb`.

One big thing this MR fixes is not being able to type a `<` or `>` anywhere. It now gets properly escaped.

This MR also adds three more custom HTML::Pipeline filters:

### AutolinkFilter

Similar to the built-in Autolink filter in that it still uses Rinku for standard http and ftp links, but then does some further processing to allow auto-linking of any URI scheme. See internal issue https://dev.gitlab.org/gitlab/gitlabhq/issues/2239

### SanitizationFilter

Created a simple custom SanitizationFilter that sub-classes the default one and adds our custom whitelisting.

### TableOfContentsFilter

Adds the anchor links to each header. This removes some processing from our Redcarpet renderer.

Closes #800, #1015, #1528, #1549

Closes GitHub [8535](https://github.com/gitlabhq/gitlabhq/issues/8535)

See merge request !584