React Flow based diagrams do not support read-only mode

[pcdavid]

See https://github.com/eclipse-sirius/sirius-web/blob/master/packages/diagrams/frontend/sirius-components-diagrams-reactflow/src/representation/DiagramRepresentation.tsx#L77

The readOnly prop passed to the representation is not read by DiagramRepresentation.

In the case of portals, where embedded representations are supposed to switch to read-only when the user is in "portal configuration edition" mode, forms (for example) are correctly disabled/read-only, but not diagrams.