WiP on selectable mosaico templates.
TODO: Make files always point to trusted URL, such that we don't have to rebase them. They are public anyway. The same goes for mosaico endpoints: /mosaico/templates and /mosaico/img
TODO:
- Allow choosing a mosaico template in a mosaico-based template
- Integrate the custom mosaico templates with templates (endpoint for retrieving a mosaico template, replacement of URL_BASE and PLACEHOLDER tags
- Implement support for MJML-based Mosaico templates
- Implement support for MJML-based templates
- Implement support for GrapeJS-based templates
