Interactive Plantuml with searchable content

[bharatrajagopalan]

This is a copy of the original pull request here

Creating this pull request on dev branch as discussed with @NGPixel

Summary:
Plantuml is currently created as an image which means that links donot work and content is treated as a picture.

This PR does the following:

• Renders plantuml as svg using object tag so that it is interactive and selectable (e.g. you can now copy paste text from within the diagram
• Places the original plantuml content inline inside the object tag so that wikijs search indexes the content for searchability.

As object is typically insecure, it will go through the following checks and sanitisation before being kept

• Only attributes that are required for plantuml are allowed.
• If any of the required attributes are missing then the node is discarded
• the data attribute must start with the URL/sub domain of preconfigured plantuml server and image type (e.g. svg) in wikijs config (e.g. https://plantuml.requarks.io/svg)
• if the above checks pass then allowed attributes other than data are reset to required values for plantuml processing

Tested using sample markdown

[bharatrajagopalan]

Closing this pull request - will squash the commits to make it more readable and open a new one