UGP support

[bbondy]

We'll want to add support to Brave Core for giving users grants from our User Growth Pool (UGP)

This is a server-side configured promotion which the browser will check for and prompt the user to engage in. Here are a few screen caps from browser-laptop:

Promotion is currently checked for using the BAT client's getPromotion method:
https://github.com/brave-intl/bat-client/blob/9c5cb329c2e007bc8ad9a69ffa59d98d95407b37/index.js#L757

Basically the grants are fetched and have an expiration date (controlled server side). The BAT client library conveniently wraps calls to the REST service on the backend.

With browser-laptop, we have a captcha in place which must be solved before a user can claim the grant:

[LaurenWags]

@NejcZdovc is testing done for this #928 (comment) sufficient to cover this issue? if not, could you suggest additional test cases? cc @kjozwiak

[NejcZdovc]

@LaurenWags yes, that should be sufficient

[LaurenWags]

Labelling as QA/No as this was basically tested under #928 (comment) as confirmed above