Add an installer route(s) and use them where possible

Problem/Motivation

Gábor Hojtsy pointed out that the final stage of Umami install where config translations are updated is quite slow - both the batch process itself, as well as the first request to the installed Drupal site.

The first request to the installed Drupal site has extra-extra cold caches, e.g. all the FileCache cached file parsing hasn't been primed before this point, and nor have compiled Twig templates, so it can be a lot longer than even the first request after a drush cr.

Additionally, the config translation import steps themselves spend a large percentage of the time compiling the installer container on every page request.

InstallerServiceProvider has the following:

// @todo Convert installer steps into routes; add an installer.routing.yml.

I think we should try to do that @todo - e.g. add either a single install.php route or a couple of routes, that are real controllers (a bit like update.php), and can use the actual runtime dependency container.

This would save compiling the container on every request and it would also potentially allow us to incrementally warm up some other caches while config translations are being imported and similar late-installer steps such as the site configure form.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet