SPA: Implement Server#disconnect

It disconnects the SPA from the server by aborting the current long polling
request.
SPA: Implement Server#signout
SPA: implement the forgetCredentials method
Worker: listen for talkilla.spa-forget-credentials messages
SidebarApp: signing out now forget the credentials and disable the SPA
Add frontend tests
HTTP: update jsdoc with return values
SPA: Server#disconnect check if there is a current xhr
Add a few frontend tests
Merge pull request #342 from tOkeshu/forget-credentials

Forget credentials