Use strings for the ApplicationSetting properties:
- restricted_visibility_levels
- default_project_visibility
- default_snippet_visibility
- default_group_visibility
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
