Move `write_alias` function to `Alias#write_to_file`

Expand the responsibility of `Alias#write_to_file` so that it performs
what it used to do plus all of what `write_alias` did.

This allows us to consolidate the functionality into a single method,
and move it into the `Alias` implementation.

The change required some modification to our `write_to_file` test:

* Create our test alias as mutable
* Write a new alias to the test file based on our test alias but with a
  different email. This allows the `write_to_file` method to work
  without erroring with an `AliasSearchError::NotFound`.
* Needed to `derive(Clone)` on `Alias` in order to be able to easily
  clone it into the new near-duplicate alias.
* Change our `unwrap()` calls to `expect()` to make it easier to see
  where exactly we panicked. Otherwise I didn't really have any way of
  knowing.
* Add some comments for clarity

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