Refactor experiments.js file

The file is growing a bit and there are opportunities to consolidate the code further and break it down for readability.

- [ ] Split the classes out into separate files
- [ ] Move the goal registration and handling functions to their own file and export
- [ ] Consolidate any duplicate code into helper functions or add them to upstream abstract class, e.g. could have an abstract `XB` class as well as the `Test` class, then the AB testing block could inherit from both `XB` and `Test`