Unify flows: Model selection / New tab

[schickling]

There are currently a lot of different ways to roughly accomplish the same. We should think through whether we could/should unify these flows:

1. Via visual query builder

2. Via new tab popup

3. Via sidebar

4. Via code-based query builder

This flow could probably be ignored for the sake of this issue though.

[sdnts]

@hrescak did this a while ago, and now we have a New Tab page. Closing!