Remove Auth::Basic and add GoogleAuth. #15132529299310

#### What's this PR do?

#### Where should the reviewer start?

`bundle install` then run it: `GOOGLE_AUTH_DOMAIN=springest.com SESSION_SECRET=3294823094802348 SECURE_KEY=239084usoeuhsrg3 bundle exec shotgun` then visit http://localhost:9393. Check that you can only log in with your springest.com google apps account.

#### These steps were taken to test this PR:
- [x] Changes are demoed to the stakeholder (_name stakeholder_)
- ~~[ ] Tested in a local development environment~~

#### Any background context you want to provide?

#### Screenshots (if appropriate)

#### Important links:
* _Asana issue(s):_ [`#15132529299310`](https://app.asana.com/0/12345678/15132529299310)
* _CodeClimate's comparison:_ [`master` ... `google-auth`](https://codeclimate.com/repos/504f5356f3ea005e1c004209/compare/google-auth)

#### Questions:
  - Do we need to translate new strings? **Yes / No**
  - Does anyone need to be notified when this has been deployed? **Yes / No**
  - Does this include migrations that might take a while? **Yes / No**