Implement the "redirect" login flow

Implement the "redirect" login flow, for cases such as using a mobile UIWebView which aren't able to use the preferred "popup" login flow.

See the specs for details:
  https://meteor.hackpad.com/OAuth-redirect-flow-spec-PeziTcaNPDP
  https://meteor.hackpad.com/OAuth-redirect-flow-part-II-vswwUKP4vXe

I extracted code to construct a URL from the `http` package into a new
`url` utility package.  The new package has no public API, it simply
has the original URL construction functions that were in `http` and
makes them available to oauth.

Fixes the Meetup account login, as Meetup now requires using
"https://api.meetup.com/2/members" instead of
"https://secure.meetup.com/2/members".

The `?close` parameter for the redirect URI is now not needed or used.
For backwards compatibility the `?close` parameter is included if the
login service configuration doesn't include the `loginStyle` field
(indicating it was created using old code).