Added global rootRequire wrapper

#### What's this PR do?
Cleans up and hopefully makes requiring our internal private modules easier. Instead of doing a bunch of `../../../` sorts of things to require modules based on their relative path, we will use `rootRequire` which starts at the app's root.

#### Where should the reviewer start?
`global.rootRequire` is added in `server.js` for when we run the app like normal, and in `_init.test.js` for when we run tests.

#### How should this be manually tested?
- `node server.js` to ensure that all module paths can be resolved
- `npm test` to make sure all those tests can still run

#### What are the relevant tickets?
Fixes #336