A cookie module

Not using ngCookie because it assumes cookies are JSONified and fails when
they’re not.
Authentication: set the xo-username
Authentication form: tests: make failure messages more precise
Authentication form: lil move-around

The handler is not relevant outside of the $scope.authenticate function, so
I’ll keep it in there.
Web UI: persist session across page reloads
Mocha: extract test eventification in its helper file

It’s not related to the runner configuration.
Tests: extract Angular mocks init into its own helper file
Cookie: replace set(name, null) with remove()
Logout button: also delete Firebase cookie