UI for Adding Content from Other Repos

@grote @NicoAlt This is the next chunk of UX design I've been focusing on, since its a likely use case for trainers. Naturally, it touches on the first use experience as well. Here's a link to the latest—https://youtu.be/dLSiaEddjc8

Feedback is welcome. I'm aiming to have these ready to start implementing on Monday (or end of day Monday).

Note: The drag and drop area in the 'explore collections' view may not stay.