Integrity check endpoint for networks

[waxlamp]

Now that the edge and node data can change under a network, we'd like to have a way for the API to check whether a network is fully consistent. This would include things like:

• ensuring that all node tables in a network actually exist
• ensuring that all edges refer to nodes that actually exist
• ensuring that the network does not refer to nodes in a table not included in the network's list of tables

We may also want to expose this in the client, perhaps via a checkmark or red X that explains what is wrong with a network.