Add notifications for affiliations.

To test:

Make sure you have redis and maildev running

1. bundle exec rails db:drop

2. bundle exec rails db:create

3. bundle exec rails db:schema:load

4. bundle exec rails db:seed:development:users

5. bundle exec rails db:seed:development:spaces

6. bundle exec rails db:seed:development:projects

7. bundle exec sidekiq -C config/sidekiq.yml

8. bundle exec rails c

9. Project.first.add_affiliation_to(Space.first) you'll see an email about a pending affiliation

10. Affiliation.first.accepted! you'll see an email about an accepted affiliation

11. Space.first.remove_affiliate(Project.first) you'll see an email about a destroyed affiliation

12. Project.first.add_affiliation_to(Space.first) you'll see another email about pending affiliation

13. Space.first.remove_affiliate(Project.first) you'll see an email about a rejected affiliation

Closes issue 781