Update email text for course attendees

[sumandari]

When we send an email to certificate recipients also send them invitation to participate in the community:

• Tell them about open days
• Tell them how to get involved
• User groups
• Etc
• A nice message from the project Chair

probably we could create a model as email text, rather than hard code it

Please see: https://github.com/kartoza/prj.app/blob/85bd0b4c8644b4bd557cd584a3399863f93f5f70/django_project/certification/views/certificate.py#L567

[Xpirix]

@jvdkwast