Change handling of Gallery's initialization sequence

- put entire ready-go-shutdown application sequence in index.php
(from bootstrap and Controller::execute())
- define two events - "gallery_ready" and "initial_request_ready"
- move all init-related code there
(from init.php files, bootstrap and Gallery::ready() code)
- remove Gallery::ready() and Gallery::shutdown() functions
- remove init.php files of rest and tag modules
- revise Theme::load_themes() to match changes
- revise Route::set() override to invert route ordering (matches module order)
- add Route::clear_all() to gallery_unittest module
- revise Controller_GalleryUnittest to clear routes and re-run "gallery_ready"
- unit tests now pass on a clean install!