The pngquant tool was used https://pngquant.org, and particularly, the
following command:
/usr/bin/pngquant -f --skip-if-larger --ext .png --speed 1 image.png
Before: 47584K
After : 34924K
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
Achilleas Pipinellis