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/db/migrate/20161201001911_add_plant_uml_url_to_application_settings.rb b/db/migrate/20161201001911_add_plant_uml_url_to_application_settings.rb
new file mode 100644
index 00000000000..b8d8742ae40
--- /dev/null
+++ b/db/migrate/20161201001911_add_plant_uml_url_to_application_settings.rb
@@ -0,0 +1,12 @@
+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class AddPlantUmlUrlToApplicationSettings < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def change
+    add_column :application_settings, :plantuml_url, :string
+  end
+end