Set agent color in UI

[blairlyons]

• [x] https://github.com/simularium/simularium-website/issues/196
• [ ] https://github.com/simularium/simularium-viewer/issues/148
• [x] https://github.com/simularium/simularium-website/issues/294

Use Case

As a user of the Viewer, I want to be able to set the color of all agents of a given type in the UI in real time.

Acceptance Criteria

• A user can choose a color and assign it to a type of agent in the Viewer UI.