follows up from #31 to offer a data model focused on formalize to the point of incorporating any data attribute types to be stored in the `panel` db.
data model [requirements](https://git.fediversity.eu/Fediversity/meta/src/branch/main/architecture-docs/data-model-requirements.md):
- [x] specifying [entity relations](https://mermaid.js.org/syntax/entityRelationshipDiagram.html#relationship-syntax) e.g. many-to-many
- [x] migrating both deployed and staged configurations
- [x] deploying of applications using the same versions
- [x] retaining relevant application state
- handling of application-specific migration logic, such as to rewrite URLs as needed - note this requirement does not affect the model here, in the sense it is handled through code rather than through the database
closes Fediversity/Fediversity#103.
Reviewed-on: #31
Reviewed-by: Valentin Gagarin <valentin.gagarin@tweag.io>
Co-authored-by: cinereal <cinereal@riseup.net>
Co-committed-by: cinereal <cinereal@riseup.net>
