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, 17 insertions, 0 deletions

diff --git a/app/views/admin/application_settings/_form.html.haml b/app/views/admin/application_settings/_form.html.haml
index 4612a7a058a..558bbe07b16 100644
--- a/app/views/admin/application_settings/_form.html.haml
+++ b/app/views/admin/application_settings/_form.html.haml
@@ -421,6 +421,23 @@
             = link_to "Koding administration documentation", help_page_path("administration/integration/koding")
 
   %fieldset
+    %legend PlantUML
+    .form-group
+      .col-sm-offset-2.col-sm-10
+        .checkbox
+          = f.label :plantuml_enabled do
+            = f.check_box :plantuml_enabled
+            Enable PlantUML
+    .form-group
+      = f.label :plantuml_url, 'PlantUML URL', class: 'control-label col-sm-2'
+      .col-sm-10
+        = f.text_field :plantuml_url, class: 'form-control', placeholder: 'http://gitlab.your-plantuml-instance.com:8080'
+        .help-block
+          Allow rendering of
+          = link_to "PlantUML", "http://plantuml.com"
+          diagrams in Asciidoc documents using an external PlantUML service.
+
+  %fieldset
     %legend Usage statistics
     .form-group
       .col-sm-offset-2.col-sm-10