Add the first set of unit tests

[akirk]

This restructures the code a little by invoking notification actions via WordPress hooks.

Then it adds unit tests for the different user engagements.

Run with composer test.

[akirk]

@amieiro I am worried about the get_users_and_first_translation_date() function. It will be very slow.

I think we should change the approach:

1. Get the unique user_ids from the translations table.
2. For each user, query:
   1. If they have their first translation date in the user option use that.
   2. Otherwise query it from the translations table and save it in a user option.
   3. If the date ends in month-day, keep the user
3. Now we have all users with an anniversary date and can sort it into years.

It will be slow on the first run but on subsequent runs only the first translation needs to be calculated for new users.