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
When rendering Asciidoc documents this merge request enables the diplay of
admonition blocks using font icons. This improves the looks of Asciidoc redered
files.
This uses the font-awesome fonts already present in Gitlab so no large
dependencies are required for this to work.
- Apply the RelativeLinkFilter filter to other formats, e.g.,
reStructuredText so links from the Files view or the Project view work
- Remove the AsciidocPipeline pipeline
Fixes#3533.
