Loading Schema Information

The UI currently pre-fetches the schema information via `GET /api/v1/:projectName/schemas`. This will result in a broken UI if the schema version is newer compared to the schema version that has been assigned to the latest branch. It can happen that schemas get updated without assigning the new schema version to the branch. The UI can in those cases display fields which are not present in the data.

The UI should instead use the `GET /api/v1/:projectName/branches/:branchUuid/schemas` endpoint to determine which schema is currently active.

Nodes which have not yet been migrated and reference a schema version that diverges from the active schema version should not be editable. Instead the option should be shown that those nodes need to be migrated.

Relates also to #242

Start by tracing the UI calls to GET /api/v1/:projectName/schemas and GET /api/v1/:projectName/branches/:branchUuid/schemas. Verify how the active branch schema is selected and how node editability is determined. Done means the branch endpoint controls the displayed schema, while nodes with divergent, unmigrated schema versions offer migration instead of editing.

Written by the indexing model from the issue text.