String-parsing in hook_custom_theme() should be replaced with theme callbacks (for now)

There are several places in Drupal 8 that currently have code like the following:

/**
 * Implements hook_custom_theme().
 *
 * @todo Add an event subscriber to the Ajax system to automatically set the
 *   base page theme for all Ajax requests, and then remove this one off.
 */
function edit_custom_theme() {
  if (substr(current_path(), 0, 5) === 'edit/') {
    return ajax_base_page_theme();
  }
}

That's incorrect, since theme callback functions should be used instead. In addition to being ugly, the code is also error-prone since other hook_custom_theme() implementations might accidentally override it, plus the string-matching might accidentally match paths it doesn't intend to.

There's an issue at #1954892: Replace 'theme callback' and hook_custom_theme() with a clean theme negotiation system that may wind up doing something to replace theme callbacks in Drupal 8 anyway. However, it's an easy patch to fix it here first, and that way the other issue can just do more of a find/replace on "theme callback" rather than dealing with these too, which if nothing else will make an eventual patch there easier to review.