Implementing use of $httpBackend to setup e2e tests

- Changing random number to a date value in "getSheets" of the service so that $httpBackend can also generate the same value so the mock is successful
- Setting up forcing picking of files with gapi.mock
- Adding a file that uses $httpBackend to mock the $http call of getting worksheets and provides fake data as a response to a published document
- Adding buttons to e2e test HTML to force pick a published and non published file