Refs #4189, more work getting UI tests to pass and adding admin screenshots, including:

- make updatetoken.php use TestingEnvironment in testmode,
- allow persisted fixture to be dropped using new --drop command line option (for ui test runner),
- fix debugging mistake in realtime map, fix merge error in empty.twig,
- make sure Fixture prints to screen only for UI test setup, re-enable CustomAlerts fixture in OmniFixture,
- make sure forced now value for realtime map is constant,
- use *.getDatabaseConfig events to override database name in TestingEnvironment
- use TestingEnvironment in proxy/piwik.php
- add wait() method to screenshot testing framework's PageRenderer
- hide current UTC time in manage websites admin screenshot