Plantuml diagram with link support
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### Description
Plantuml provide a way to integrate link into diagram that can make diagram more interactive.
### Proposal
Integrate the links with the two methods :
When the specified format is png, append the map data provided and integrate the usemap attribute to the `<img>` tag.
When the specified format is svg, use the `<svg>` data from the svg link instead of putting the url into a `<img>` tag.
By default use the png and if the map data is not available go to the existing behavior.
### Links / references
Description about link in plantuml : http://plantuml.com/link
Example of diagram with links : http://www.plantuml.com/plantuml/uml/XL6zZi8m3Dxp51OxxSw0W6TWEI66q7ERwDouLxjYGBpxEGaI4pqfW-Dy_dwSWwYVTHd9-KvvXXFVu789gjEkRIVoARyr7O-juE-2gSFhzGNTd4CKiBEEXoF5F0LbQWDUQiTbgUs-NZvnP7CskeE0HEmIAm9Xh-ju1-2XOEzHWzKKqu_3z5sQhYgQT7Rmegx85TR7cPaqJj1Rz155_81F41wni9je4PpZ4FJVDDwm0UTgvnsA-sp9VV42dK-moEfUYgvzG2eqaZTCmiqxvy4gc_W3
Map data link of the example : http://www.plantuml.com/plantuml/map/XL6xhi8m3Dtp5HOxT6T0WczWGOZ1jxWamerBxUeEY7yd3mGJHSgGMEVfO6iomax-u7ZqPiEwQGR6Q5FFgrRwHkasKMpfT79Oe8kz3mhfpEDXoo6peB0MW5edBbijD_NnJSDmj4HTWoTcSL6CWAcpURm3G2ZO_s0-tJd4gwDuBaDN7PDrJl2IhiWPrVjUX2qCqANgFQdYXHwWqbDHJwR5S0mNRp_ROC9w6ckSJodfSYHta0bQZ33fx5wAR_g0M6GqRvXLLk-UJm00
Svg link of the example : http://www.plantuml.com/plantuml/svg/XL6xhi8m3Dtp5HOxT6T0WczWGOZ1jxWamerBxUeEY7yd3mGJHSgGMEVfO6iomax-u7ZqPiEwQGR6Q5FFgrRwHkasKMpfT79Oe8kz3mhfpEDXoo6peB0MW5edBbijD_NnJSDmj4HTWoTcSL6CWAcpURm3G2ZO_s0-tJd4gwDuBaDN7PDrJl2IhiWPrVjUX2qCqANgFQdYXHwWqbDHJwR5S0mNRp_ROC9w6ckSJodfSYHta0bQZ33fx5wAR_g0M6GqRvXLLk-UJm00
### Documentation blurb
#### Overview
The benefits can provide a more powerful diagram in order to create for example link to another wiki page that display another diagram and so on..
it's for every user that want to create diagram with plantUML in gitlab.
#### Use cases
Create a plantuml diagram with links and integrate them in a wiki.
### Feature checklist
Make sure these are completed before closing the issue,
with a link to the relevant commit.
- [ ] [Feature assurance](https://about.gitlab.com/handbook/product/#feature-assurance)
- [ ] Documentation
- [ ] Added to [features.yml](https://gitlab.com/gitlab-com/www-gitlab-com/blob/master/data/features.yml)
issue