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, 4 insertions, 0 deletions

diff --git a/spec/lib/gitlab/asciidoc_spec.rb b/spec/lib/gitlab/asciidoc_spec.rb
index f3843ca64ff..ba199917f5c 100644
--- a/spec/lib/gitlab/asciidoc_spec.rb
+++ b/spec/lib/gitlab/asciidoc_spec.rb
@@ -8,6 +8,10 @@ module Gitlab
     let(:html) { 'H<sub>2</sub>O' }
 
     context "without project" do
+      before do
+        allow_any_instance_of(ApplicationSetting).to receive(:current).and_return(::ApplicationSetting.create_from_defaults)
+      end
+
       it "converts the input using Asciidoctor and default options" do
         expected_asciidoc_opts = {
             safe: :secure,