Use add facebook endpoint

- in facebook claim reward flow, always get a new auth token from the user (FB.login). if the user hasn't linked their fb account previously, link it and then claim. otherwise, update the auth token and then claim
- brings in bower component angular-facebook and deletes util/fb.js
- cleans up facebook-reward-controller.js, makes the functions promiseable
I think this should be changed to:
```js
if (response.authResponse) {
```
[FB docs](https://developers.facebook.com/docs/reference/javascript/FB.login/v2.1). [Sentry bug](https://app.getsentry.com/gostellarorg/stellar-client/group/27420152/) in removed `fb.js` code.