Change how the plotting config is represented

[rneswold]

Right now, any tweak to the plot config requires changes to -core and the GraphQL layer. We also want these configs to be persistent, so we'd be making table changes in Postgres, too! This is too much work and makes minor config changes in the plotting app take too long before they can be tested and used.

This issue tracks the effort to simply save the configuration as a string:

• The Flutter app can take its PlotConfigSnapshot class and serialize it as a JSON string (using the .to_json() and .from_json() API)
• The GraphQL service can deprecate and remove the PlotConfigSnapshot types and just save the string value. This change prevents clients from requesting a subset of the configuration, but I doubt that was something anyone was going to do anyway.
• There has been some questions about why the plot config API is in the same schema as retrieving device data. We should discuss whether there should be a separate endpoint for a plot config schema.