immutable: true|false for 0.97

Graph tooling 0.97.0 enforces the use of immutable, even for things where immutable is false. This would mean all schema files should be changed to have the explicit immutable field.

cfr: https://github.com/Amxx/graphprotocol-utils/issues/15
cfr: https://github.com/graphprotocol/graph-tooling/pull/1994