Allow a file system to be set as a default in the UI

[josevalim]

We should also improve the onboarding experience here. Once I set the file system, we should show users where they can change the filesystem. Maybe we can have a confirmation page telling them where to click? We should probably build this after we do #633.

[hameedhub]

@josevalim is this task available?

[josevalim]

Yes! We show filesystems in the Settings page. We can add a button that says "make default" to each or similar. :)