Add support for embeddings using the neo4j and solr vector stores

[jamesamcl]

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This PR adds a new API endpoint /similar which returns similar classes from the embedded vector store, and a new "similar classes" section in the UI.

Neo4j and Solr are updated to the latest versions to enable use of the vector store.

This PR also incorporates some other changes as it was easier to rebase everything into the same tree for testing:

• A new "linksTo" API endpoint is added to search for entities which link to other entities (#877)
• It is now possible to pass an OWL file instead of a JSON config if you just want to load an ontology without having to make a config file (useful for testing)