ds-routing refactor and tests

#### What's this PR do?
More effectively decouples our custom Mobile Commons routing code into a library. So now, if we wanted to remove the entire thing from the app, we could just remove the follwoing line in router.js: `require('./lib/ds-routing')(app);`. Also adds tests for the functions in `ds-routing`.

#### Where should the reviewer start?
Migrating code over was relatively straightforward with `MCRouting`, `Tips`, and their relevant config and model files being moved over to the `lib/ds-routing` folder. Tests are all in `test/ds-routing.js`.

#### How should this be manually tested?
As long as `npm test` works, we should be good. But I also manually test the endpoints using Postman.

#### Any background context?
This should be reviewed after #286 and #287

#### What are the relevant tickets?
Closes #31 
Sets us up to do #236