Fix/allow custom properties to pass on update for links

[danielcaldas]

This PR fixes the following:

• Allows custom properties to be freely passed down on links.
• Fixes the bug where removing a custom color from a link after adding it does not update the Graph.
• Avoids ticking the graph when a link custom property such as the link.color is updated.
• Adopts the same whitelist strategy to map client links data as the one used for nodes.
• Adds the transform-object-rest-spread babel plugin replacing some usage of Object.assign by object spread operator.
• Adds some more steps to e2e tests to ensure correct link updating behavior.