Simplify Online Repos Installation Workflow Steps

This is part of#903 (Make the installation process shorter).

In the installation workflow (#914), we have these steps:

Asking about Online Repos

Selecting Online Repos

Problems

• It's two steps (three if you also count the "Writing..." step where the user just has to wait)

• One step would be totally sufficient (see below)

• Most information in the second step is redundant (see below)

• The second step is much too detailed. It suggests much more complexity than there really is.

• Most users simply don't care: Just use the defaults.