revisit placeholder mechanism

[fbiville]

The generated code samples currently use placeholders for the connection parameters (see this example).
This is in turn used by the Sandbox UI to replace these placeholders with the connection settings of the actual provisioned sandbox instance.
The downside of this is that the code is not directly runnable by users, as they need to replace the placeholders as well.

We might want instead to generate valid connection settings, which implies that:

• a demo server database is always available for each sandbox repository in https://github.com/neo4j-graph-examples/
• the instance data matches the dump versioned in the corresponding sandbox repository
• sandbox UI (and Neo4j Desktop UI at some point) is able to replace these values with the actual connection settings of the running instance (e.g. replacing "neo4j+s://demo.neo4jlabs.com:7687" and replacing "pole", "pole" with "neo4j","a-sandbox-password" and also change the "pole" database bit in the session to "neo4j" in sandbox 4.x)
• the values are properly documented so users are not confused