Improve mailing feature with localized emails

[spaenleh]

We would like to be able to send localized emails to users.

We will create a mailing entity, the mailing entity will have:

• name
• audience
• timestamps

We will link to this mailing entity a localized_emails entity:

• subject
• message
• button_text (nullable)
• button_url (nullable)
• language

mailing has a many relation ship to localized_emails

The audience is a string that represents a way to get members, for example: active_users, h5p_creators, french_active_users ...
It is resolved when we send the email, fetching the list of emails that it represents.