User curve model

[louispt1]

This PR:

• Adds a migration to introduce the user curve schema
• Introduces the user curve model and associates it with a scenario
• Ports curve and metadata methods from the ScenarioAttachment model to the UserCurve model
• Introduces MeritCurveSerializer to serialize/deserialize curves directly into and out of Merit::Curve objects
• Introduces spec for the model and serializer
• Adjusts attachment.curve method calls to refer instead to the UserCurves model
• Refactors spec related to custom curves on scenario, inputs, custom curve collection

Note: I compared the size of user curves serialized this way vs the csv files and the MessagePacked versions are larger. Depending on the data, the CSV blobs tend to be around 50kb and the MessagePack blobs tend to be 70kb. Obviously there are other advantages such as speed of read/write and dealing with floating points more effectively, but I thought it would be worth noting that finding here!

Still to implement:

• Changes to the custom curves controller (csv / json response), custom curve serializer and related spec and methods.
• Attach / detach curve services.

Closes: #1519 and #1520 and #1518