MDL-43422 JavaScript: Break moodle-core-util into separate files

This is intended to ease future maintainability of each component of the
former javascript-static.js.
MDL-43422 JavaScript: Fix minor issues with M.util.get_string
MDL-43422 JavaScript: Fix minor issues with M.util.image_url
MDL-43422 JavaScript: Fix minor issues with legacy JS
MDL-43422 JavaScript: Fix minor issues with focus_login_form
MDL-43422 JavaScript: Fix minor issues with init_maximised_embed
MDL-43422 JavaScript: Fix minor issues with toggle_class_on_click