Add support for PlantUML diagrams in Asciidoc.

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

1 files changed, 12 insertions, 0 deletions

diff --git a/lib/gitlab/asciidoc.rb b/lib/gitlab/asciidoc.rb
index fa234284361..0618107e2c3 100644
--- a/lib/gitlab/asciidoc.rb
+++ b/lib/gitlab/asciidoc.rb
@@ -1,5 +1,6 @@
 require 'asciidoctor'
 require 'asciidoctor/converter/html5'
+require "asciidoctor-plantuml"
 
 module Gitlab
   # Parser/renderer for the AsciiDoc format that uses Asciidoctor and filters
@@ -29,6 +30,8 @@ module Gitlab
       )
       asciidoc_opts[:attributes].unshift(*DEFAULT_ADOC_ATTRS)
 
+      plantuml_setup
+
       html = ::Asciidoctor.convert(input, asciidoc_opts)
 
       html = Banzai.post_process(html, context)
@@ -36,6 +39,15 @@ module Gitlab
       html.html_safe
     end
 
+    def self.plantuml_setup
+      Asciidoctor::PlantUml.configure do |conf|
+        conf.url = ApplicationSetting.current.plantuml_url
+        conf.svg_enable = ApplicationSetting.current.plantuml_enabled
+        conf.png_enable = ApplicationSetting.current.plantuml_enabled
+        conf.txt_enable = false
+      end
+    end
+
     class Html5Converter < Asciidoctor::Converter::Html5Converter
       extend Asciidoctor::Converter::Config