Add signin link again, add signout functionality again and fix e2e specs for authentication

Summary:
* background: the new collapsible user menu introduced with D491 (c06fdd988e10801a7c50d9ad55837b94dff55d24) broke the authentication user flow
  * the signin link was missing completely (at least it was not visible)
  * the signout link under the collapsible user menu had no effect
* replace the signin-signout-button directive with two new directives
  * signin-button
  * user-menu
  * make sure they are only showed when signed in / signed out
  * adapt/fix the authentication e2e specs

Test Plan:
* start server
  * e.g. 'gulp --node-server'
* check that
  * signin link is present when signed out
  * user menu is present when signed in
* run the specs
  * protractor test/protractor-conf.js

Reviewers: shadowhand

Reviewed By: shadowhand

Subscribers: aMoniker, seth, rjmackay

Differential Revision: https://phabricator.ushahidi.com/D495