Without setting outfilesuffix Asciidoctor defaults to .html. When .html is used,
inter-document cross references do not work in Gitlab's repository / file renderer.
See more information from
http://asciidoctor.org/docs/user-manual/#navigating-between-source-files
and
http://asciidoctor.org/docs/user-manual/#inter-document-cross-references

The initializers including this were doing so at the top level, so every object
loaded after them had a `current_application_settings` method. However, if
someone had rack-attack enabled (which was loaded before these initializers), it
would try to load the API, and fail, because `Gitlab::CurrentSettings` didn't
have that method.

To fix this:

1. Don't include `Gitlab::CurrentSettings` at the top level. We do not need
   `Object.new.current_application_settings` to work.
2. Make `Gitlab::CurrentSettings` explicitly `extend self`, as we already use it
   like that in several places.
3. Change the initializers to use that new form.

Robert Speicher authored 7 years ago and Bob Van Landuyt :neckbeard: committed 7 years ago

(security-9-1) Render asciidoc & other markup using banzai in a pipeline

See merge request !2098

Because the post-processing of the rendered README is dependent on the
context (i.e. the current user), do the post-processing when the
README is being displayed.

Add sanitization filter to asciidocs output to prevent XSS

See https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/2057

This MR enables rendering of PlantUML diagrams in Asciidoc documents. To add a
PlantUML diagram all we need is to include a plantuml block like:

```
[plantuml, id="myDiagram", width="100px", height="100px"]
--
bob -> alice : ping
alice -> bob : pong
--
```

The plantuml block is substituted by an HTML img element with *src* pointing to
an external PlantUML server.

This MR also add a PlantUML integration section to the Administrator -> Settings
page to configure the PlantUML rendering service and to enable/disable it.

Closes: #17603

When rendering Asciidoc documents this merge request enables the diplay of
admonition blocks using font icons. This improves the looks of Asciidoc redered
files.

This uses the font-awesome fonts already present in Gitlab so no large
dependencies are required for this to work.

- Apply the RelativeLinkFilter filter to other formats, e.g.,
  reStructuredText so links from the Files view or the Project view work
- Remove the AsciidocPipeline pipeline

Fixes #3533.