Collapsable Regions and Pasting Images

[dmc6297]

1. Create a collapsable region

<markdown collapsable title="my image.png" type="Image">
</markdown>

2. Allow pasting of images. Converted to base64 and embedded into markdown as <img>

[R0Wi]

I tested this feature and it works great, thank you @dmc6297 !

Screencast.mp4

Maybe the code in the area of https://github.com/requarks/wiki/pull/6551/files#diff-6572222f98358241abe369388929bd7692fc070e1ef397ec605ee31f83f984d0R684 could be refactored a little bit for better readability. But from a functional point of view this is exactly what people need in #5441. Hope some of the maintainers will find some time soon to review this 🚀