Document uploading about bonsai

[impworks]

Haven't found any library for PDF rendering that is both open source and supports .NET Core. This JS library is the most promising one so far:

https://github.com/mozilla/pdf.js

It shouldn't be a problem since we already use NodeServices for ELK.js tree layout calculation. However, I couldn't get the example running on my device out-of-the-box. Needs further experiments and investigation.

[impworks]

The idea of generating document previews was ultimately scrapped. The options were:

1. pdf.js: requires way too many dependencies and bloats the container (needs native canvas, which must be built from sources, which requires build-essentials etc)
2. iText7: cannot use legally without converting Bonsai to AGPL
3. ImageMagick: uses GhostScript for PDF rendering, which is also AGPL

The impact of the feature is way to small to make tackling those troubles reasonable. So this is currently implemented with a static stub image saying "PDF".

Should a lightweight and free PDF rendering library appear, this issue can probably be revisited.