Add browser-based authentication method

[leplatrem]

See https://bugzilla.mozilla.org/show_bug.cgi?id=1630651
This will allow us to publish data from mozilla-central using ./mach.

This works as expected:

>>> import kinto_http
>>> c = kinto_http.Client(server_url='https://remote-settings-dev.allizom.org/v1', auth=kinto_http.BrowserOAuth())
>>> c.server_info()["user"]

I wonder if there is a less hacky way to accomplish this. Any idea?

[alexcottner]

This looks reasonable to me. We might want to adjust our coverage requirements.

[alexcottner]

Nice