Refactor EmailTemplate relations

[mipyykko]

Currently the email template relations are directly specified in the tables, so that, for example, course has three different fields for templates of varying types.

As the template types and uses for the templating system seems to grow and the current implementation is not very flexible, it would be beneficial to refactor this.

An idea would be to have a many-to-many relation:

• the join table could have fields for whatever might be using the template, so in addition to the email_template_id, one or more of course_id, completion_id, course_stats_subscription_id, organization_id and so on
• the templates itself would have the types as they currently do, perhaps converted to enums * the triggers should be refactored accordingly, so as we currently check if a course has a completion_email set, we would see if there's a template with the type of completion associated with the course -- the same would apply to other types

This shouldn't be too much of a job (famous last words.)