Testing: Add FactoryBot to project

[epintozzi]

Add FactoryBot to this project. I think it will help us write tests more efficiently and easier to read.

I'd start with a PR to add the basic setup - don't attempt to rewrite and refactor all the tests. Let's just get the gem installed and setup ready to go. This PR should also include a brief summary in the README that the expectation is to use FactoryBot when possible, as appropriate moving forward.

Create followup issues for each individual spec file and we can replace with factories little by little.