feat: jumar notification module

[btkostner]

This moves notification logic into it's own jumar_notification module. This is because it uses templates and that would be weird placing in the regular jumar namespace. Needs some room to grow.

[btkostner]

Things to consider:

• Opening email in browser built in vs from third party service
• Using mjml to template emails https://github.com/akoutmos/mjml_eex