[meta] Streamline the UI installer

Problem/Motivation

The current flow of the installer is something like this:

1. Language and database
2. Profile selection
3. Long batch installing the profile + modules + config
4. (if multilingual) batch to update translations
4. Site information form
5. (if multilingual) another batch to update config translations

It is not a bad experience, but it could be better, and improvements to it could help elsewhere in core too.

I think we can make it this instead:

1. Language and database
2. Project browser - installing modules and recipes as they're chosen
3. Site information form

Steps to reproduce

Proposed resolution

To enable this, we'd need the following:

1. Remove install profile selection from the installer (to start with, we could add a skip button)
2. Add project browser to the installer (requires project browser in core)
3. Refactor or extend batch API to allow a 'batch' element' to run down batches (and maybe queues) via AJAX, without an interstitial page or redirect. This would allow translations and any other batches triggered as a result of module install to be run without (entirely) blocking the end user.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet