Meteor.loginWith<External> now waits for config

While this could mean that there's an indefinite wait before logging you
in, any condition where login configuration isn't being swiftly
published (after all, it should be cached) is a bigger problem.

`Accounts`/accounts-base is an awkward place to put this, because in
theory we wanted the packages like "twitter" to not depend on
accounts. But it turns out that service-configuration depends on
accounts anyway, so this isn't much of a regression. We can't put this
function in service-configuration because that package doesn't actually
do the subscription and make the ready handle. (Maybe oauth would be a
better place, but again, that's a package that doesn't currently depend
directly on accounts-base and would have to.)

Fixes #1911 and #2048.