Make the network JSON uploader more flexible and allow JSON table uploads

[JackWilb]

Does this PR close any open issues?

Closes #124

Give a longer description of what this PR addresses and why it's needed

This makes it so json networks that define either _key or id as node identifier and either _from and _to or source and target can be uploaded. This means that it's possible to upload data as we use it in mulinet or as it is used in D3. Another benefit of this is that data that is downloaded from the client is now upload-able again.

I'm also planning to enable JSON uploads of tables so that the downloaded versions of the tables can be uploaded.

Provide pictures/videos of the behavior before and after these changes (optional)

N/A

Are there any additional TODOs before this PR is ready to go?

TODOs:

• Update relevant documentation (Document accepted data formats for both tables and networks multinet-docs#33)
• Enable JSON table uploads
• Update d3 json uploader test
• Add json table uploader tests