Themes are now plugins, this unifies the intallation experience and
loading syntax
Starting to re-do theme assets handling
Relocating theme assets and updating tests
Fixing HtmlHelper tests
Fixing Helper tests
Fixing tests in dispatcher filters related to Themes
Fixing remaining tests related to themes
And... done fixing the tests
Fixing CS errors
Small style corrections
Merge pull request #3537 from cakephp/3.0-plugin-as-themes

3.0 plugin as themes