Create tester repo as a private repo

As a measure against plagiarising bug reports, we can create the tester repo as a private repo initially, and ask if the user wants to make it public when the user exits CATcher.

Things to consider: does this need higher permissions from the user?

Start by tracing where CATcher creates the tester repository and how GitHub permissions are requested. Determine whether private repository creation requires higher permissions, then verify how asking the user at CATcher exit should make the repository public. Done means the permission implications and exit-time behavior are defined and implemented.

Written by the indexing model from the issue text.