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, 8 insertions, 2 deletions

diff --git a/doc/api/settings.md b/doc/api/settings.md
index 0bd38a6e664..f86c7cc2f94 100644
--- a/doc/api/settings.md
+++ b/doc/api/settings.md
@@ -44,7 +44,9 @@ Example response:
    "repository_storage": "default",
    "repository_storages": ["default"],
    "koding_enabled": false,
-   "koding_url": null
+   "koding_url": null,
+   "plantuml_enabled": false,
+   "plantuml_url": null
 }
 ```
 
@@ -80,6 +82,8 @@ PUT /application/settings
 | `koding_enabled` | boolean | no | Enable Koding integration. Default is `false`. |
 | `koding_url` | string | yes (if `koding_enabled` is `true`) |  The Koding instance URL for integration. |
 | `disabled_oauth_sign_in_sources` | Array of strings | no | Disabled OAuth sign-in sources |
+| `plantuml_enabled` | boolean | no | Enable PlantUML integration. Default is `false`. |
+| `plantuml_url` | string | yes (if `plantuml_enabled` is `true`) |  The PlantUML instance URL for integration. |
 
 ```bash
 curl --request PUT --header "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" https://gitlab.example.com/api/v3/application/settings?signup_enabled=false&default_project_visibility=1
@@ -112,6 +116,8 @@ Example response:
   "container_registry_token_expire_delay": 5,
   "repository_storage": "default",
   "koding_enabled": false,
-  "koding_url": null
+  "koding_url": null,
+  "plantuml_enabled": false,
+  "plantuml_url": null
 }
 ```