Persist projects and artifacts

[ekuiter]

Right now, all data is kept in RAM by the server. Restarting the server resets all data.

We have to think about storing data as files, or in a database, or in AWS. (Also consider GDPR!)
Still, it should be possible to use the editor without any persistence mechanism (just load some data temporarily).

[ekuiter]

When persisting feature models, the identifying information for features has to be saved as well, i.e. the UUID, so we have the UUIDs when loading a feature model. This also implies that FeatureIDE's getInternalId is NOT suitable as a UUID, because it cannot be modified.