Template Editor

[wolflu05]

This issue is a collection of improvements for the Template editor introduced with #6541:

Ideas

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1. add plugin interface to let plugins provide different editors/previews (to let plugins provide e.g. drag and drop editors)
2. easily upload report assest via drag and drop in code editor
3. add ability to edit report snippets (maybe also the ability to switch between them using tabs)
4. add custom template filters api form field component to visually edit template filters
5. codemirror infer available django context variables
6. add vertical resize drag handler
7. ~~add printcss preview type (https://printcss.live/)~~=> can be a plugin now
8. define a custom language for django templates to be more accurate (example: https://github.com/codemirror/lang-liquid)
9. ??? maybe switch to monaco ???
10. allow previewing without saving the template (ref: Refactor tables and API for reports and labels #5841 (comment))
11. show warning if there are unsaved changes before closing the drawer

[matmair]

@wolflu05 with the changed UI customisation options of PUI - have any of the items above changed?

[wolflu05]

Thanks for the reminder. I have updated the todo list in the initial comment.