refactor(client): Reduce fxa-js-client fetch boilerplate (#4090)
refactor(client): Reduce fxa-js-client fetch boilerplate (#4090)

In lib/fxa-client.js, add a `withClient` decorator function that
gets a fxa-js-client instance and passes it as the first parameter
to a function. This allows a lot of `this._getClient().then...` boilerplate
to be removed.

r=@vbudhram
mozilla/fxa-content-server,shane-tomlinson/fxa-content-server,mozilla/fxa-content-server,shane-tomlinson/fxa-content-server,mozilla/fxa-content-server,shane-tomlinson/fxa-content-server