Bug 897600 Add firefox accounts screen to FTE flow r=arcturus,ferjm

This patch let us to sign in/sign up in Firefox Accounts during FTE. The patch contains a pref which let us to disable or enable this step in the FTU (Currently it's disabled). For testing update the property `identity.fxaccounts.ui.enabled` in  `build/config/common-settings.json` to `true`