Fixing UI tests build:

- change idsite to idSite in Overlay URLs in piwik.js
- don't init top controls if top controls container can't be found
- add default parameter for $urls in SitesManager.setSiteAliasUrls so site alias URLs can be reset
- fix bugs in UI tests database setup (including moving dashboard setup to UITestFixture class & keeping outside of
  setUp function so --persist-fixture-data will work in case the super user is deleted)
- Add queryParamOverride testingEnvironment variable.
- fix screenshot testing page renderer's method of waiting for CSS images
- add margin bottom & left for widgetized controls for tests
- add diagnostics to upload_artifacts.sh script