Complete platform-client for Blip integration

This should complete the migration of API calls in Blip codebase to `platform-client`. Latest additions:

- Add `getDeviceDataForUser()` endpoint
- Add `processDeviceData()`, with RxJS data processing
- Add `getUserId()` convenience method, for current logged-in user
- Add `getUploadUrl()`

@cheddar, would you care to a look and see if this is more or less what you had in mind? Thanks!