Convert this repo into the new AB-3 repo.
Compared to the old AB-3, this repo is derived from the AB-4 codebase,
and thus shares all of its improvements, and allows us to more easily
port over changes from AB-4 in the future.
Checkout this PR version locally
```sh
git fetch https://github.com/se-edu/addressbook-level35.git refs/pr/19/1/head:BRANCHNAME
```
where `BRANCHNAME` is the name of the local branch you wish to fetch this PR to.
Fixes #2