[NcRichContentEditable] Browser formatting shortcuts format the input field content but not the value

[ShGKme]

Web-browsers (at least chromium-based) have formatting shortcuts like Ctrl+B (for unknown reasons).

See: https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/editing/editing_behavior.cc;l=141-148;bpv=0;bpt=1

This formatting changes HTML content but doesn't change the actual NcRickContenteditable value.

Proposals:

1. Simple - block these shortcuts to not confuse users
2. Simple + MD support — handle shortcuts to add markdown formatting (like in GitHub)
3. Complex - parse HTML formatting. This would allow pasting more complex formatted text but probably has a huge list of edge cases

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Originally from nextcloud/talk-desktop#907