Issue 111 - make email required on backend

[lpatmo]

[x] Change the line in the model to make sure email is required
[x] Make migrations file and run migrations
[x] Make sure tests pass
[x] Double checked in Postman that if I don't include an email, the request is not successful

Implementation details

One-line change:

Migration commands I ran:
docker-compose run --rm app ./manage.py makemigrations --check and ocker-compose run --rm app ./manage.py migrate

Postman:

The tests pass:

I double checked and used the docker commands from @chris48s' codebuddies#109, so am basing this PR off of that PR.

[lpatmo]

Note: this is for documentation purposes only (we don't want to merge this into the testing PR necessarily), so am going to close but keep on the record!