Send an email for newly enrolled members

[supertom01]

Please add the following information to your pull request:

Please describe what your PR is fixing
When a new member is enrolled, they will receive an email.

Concretely, which issues does your PR solve? (Please reference them by typing Fixes/References Inter-Actief/amelie#<issue_id>)
Fixes #720

Does your PR change how we process personal data, impact our privacy document, or modify (one of) our data export(s)?
no

Does your PR include any django migrations?
no

Does your PR include the proper translations (did you add translations for new/modified strings)?
yes, however after the email template has been updated the translations should be added

Does your PR include CSS changes (and did you run the compile_css.sh script in the scripts directory to regenerate the compiled.css file)?
no, my PR does not include CSS changes

Does your PR need external actions by for example the System Administrators? (Think about new pip packages, new (local) settings, a new regular task or cronjob, new management commands, etc.)?
no

Did you properly test your PR before submitting it?
no, testing the entire mail system is quite hard due to the need to enroll members and finalize all of the steps. This feature has only been tested for creating external members.

[supertom01]

@[email protected] @SimplyPancake asked me to let you write the invitation email. You should be able to find the template for it within new_member.mail. Please let me know when you've updated it (and provide a dutch translation too :))