Adding new source

[johnlafleur]

When in "Sources", user can click on "+ New source" in the header. This should start the adding new source flow, which is very similar to the onboarding (apart from the step of adding a destination, which doesn't exist here):
https://www.figma.com/file/AODY4oi15iI5l0fbxHvobt/Daxtarity?node-id=5782%3A45752

After adding a new source, the user should be redirected to the "Sources" section.

@cgardens