fix(signin): choosing to use a different account clears cached credentials

Fixes #1721.
Merge pull request #1767 from mozilla/issue-1721-choose-different-sign-in-2

fix(signin): choosing to use a different account clears cached credentials

I really dig the simplicity of this solution. r+

Thanks @zaach!
tests(functional): disable sync ui tests
Merge pull request #1770 from vladikoff/disableFuncSyncTest

tests(functional): disable sync ui tests
Merge pull request #1756 from mozilla/issue-1755-more-functional-tests

feat(test): Add more functional tests sign up/reset password flows.
fix(tests): remove about:prefs tests from full testing
feat(client): Start to add a broker!
remove isAutomatedBrowser
* Remove forceAuth from Session, add isForceAuth to the relier
* Pass the broker to all views.
* Add a test for the confirm_reset_password page to ensure XSS emails do not make it through.
remove unused isForceAuth
Create the Redirect and WebChannel Brokers. Get rid of the Redirect Channel. Move the functionality relating to the WebChannel from ServiceMixin to the WebChannelBroker.
Move more WebChannel functionality from Channels.js to WebChannel
Push FxaDesktop related code from fxa-client to the FirefoxDesktopBroker.
Remove the Channels abstraction, it is now taken care of by the Brokers
fix web channel verification.