Bug 957425 - add about:sync-setup for entry-point into Fxa-based sync. r=ttaubert?
first-cut at having sync preferences magically do the right thing if Fxa is enabled or not
Have bid_identity not return a status of LOGIN_FAILED_NO_PASSPHRASE if we are still waiting for the passphrase from the token server
first cut at setting the identity.fxaccounts.enabled pref automatically on fxa signin
Make browserid_identity at first-class identity.

This patch tries to do 2 things:

* When sync initializes, determine the identity manager to use.  Currently the
  default identity manager is created and used, then replaced when we determine
  to use Fxa.  Now we create the correct one from the outset, and it never
  changes, except...

* ... A sync "reset" resets this too - so upon reset we can swap from the
  legacy provider to the Fxa provider.

This is all based on existing sync prefs and the 'identity.fxaccounts.enabled'
preference.