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

2 files changed, 87 insertions, 0 deletions

diff --git a/doc/administration/img/integration/plantuml-example.png b/doc/administration/img/integration/plantuml-example.png
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diff --git a/doc/administration/integration/plantuml.md b/doc/administration/integration/plantuml.md
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+# PlantUML & GitLab
+
+> [Introduced][ce-7810] in GitLab 8.16.
+
+When [PlantUML](http://plantuml.com) integration is enabled and configured in
+GitLab we are able to create simple diagrams in AsciiDoc documents created in
+snippets, wikis, and repos.
+
+## PlantUML Server
+
+Before you can enable PlantUML in GitLab; you need to set up your own PlantUML
+server that will generate the diagrams. Installing and configuring your
+own PlantUML server is easy in Debian/Ubuntu distributions using Tomcat.
+
+First you need to create a `plantuml.war` file from the source code:
+
+```
+sudo apt-get install graphviz openjdk-7-jdk git-core maven
+git clone https://github.com/plantuml/plantuml-server.git
+cd plantuml-server
+mvn package
+```
+
+The above sequence of commands will generate a WAR file that can be deployed
+using Tomcat:
+
+```
+sudo apt-get install tomcat7
+sudo cp target/plantuml.war /var/lib/tomcat7/webapps/plantuml.war
+sudo chown tomcat7:tomcat7 /var/lib/tomcat7/webapps/plantuml.war
+sudo service restart tomcat7
+```
+
+Once the Tomcat service restarts the PlantUML service will be ready and
+listening for requests on port 8080:
+
+```
+http://localhost:8080/plantuml
+```
+
+you can change these defaults by editing the `/etc/tomcat7/server.xml` file.
+
+
+## GitLab
+
+You need to enable PlantUML integration from Settings under Admin Area. To do
+that, login with an Admin account and do following:
+
+ - in GitLab go to **Admin Area** and then **Settings**
+ - scroll to bottom of the page until PlantUML section
+ - check **Enable PlantUML** checkbox
+ - set the PlantUML instance as **PlantUML URL**
+
+## Creating Diagrams
+
+With PlantUML integration enabled and configured, we can start adding diagrams to
+our AsciiDoc snippets, wikis and repos using blocks:
+
+```
+[plantuml, format="png", id="myDiagram", width="200px"]
+--
+Bob->Alice : hello
+Alice -> Bob : Go Away
+--
+```
+
+The above block will be converted to an HTML img tag with source pointing to the
+PlantUML instance. If the PlantUML server is correctly configured, this should
+render a nice diagram instead of the block:
+
+![PlantUML Integration](../img/integration/plantuml-example.png)
+
+Inside the block you can add any of the supported diagrams by PlantUML such as
+[Sequence](http://plantuml.com/sequence-diagram), [Use Case](http://plantuml.com/use-case-diagram),
+[Class](http://plantuml.com/class-diagram), [Activity](http://plantuml.com/activity-diagram-legacy),
+[Component](http://plantuml.com/component-diagram), [State](http://plantuml.com/state-diagram),
+and [Object](http://plantuml.com/object-diagram) diagrams. You do not need to use the PlantUML
+diagram delimiters `@startuml`/`@enduml` as these are replaced by the AsciiDoc `plantuml` block.
+
+Some parameters can be added to the block definition:
+
+ - *format*: Can be either `png` or `svg`. Note that `svg` is not supported by
+   all browsers so use with care. The default is `png`.
+ - *id*: A CSS id added to the diagram HTML tag.
+ - *width*: Width attribute added to the img tag.
+ - *height*: Height attribute added to the img tag.
+