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, 6 insertions, 0 deletions

diff --git a/app/models/application_setting.rb b/app/models/application_setting.rb
index bf463a3b6bb..8fab77cda0a 100644
--- a/app/models/application_setting.rb
+++ b/app/models/application_setting.rb
@@ -68,6 +68,10 @@ class ApplicationSetting < ActiveRecord::Base
             presence: true,
             if: :koding_enabled
 
+  validates :plantuml_url,
+            presence: true,
+            if: :plantuml_enabled
+
   validates :max_attachment_size,
             presence: true,
             numericality: { only_integer: true, greater_than: 0 }
@@ -184,6 +188,8 @@ class ApplicationSetting < ActiveRecord::Base
       akismet_enabled: false,
       koding_enabled: false,
       koding_url: nil,
+      plantuml_enabled: false,
+      plantuml_url: nil,
       repository_checks_enabled: true,
       disabled_oauth_sign_in_sources: [],
       send_user_confirmation_email: false,