Add support for distribution clean installations

Problem/Motivation

When the Gin theme is used as an admin theme for a clean site install. It tries to make calls to API that might not be available to the system at the time, e.g. site configurations or user entities/roles.

Steps to reproduce

Use Gin for a clean site install, e.g. adding the following to the distribution info.yml:

distribution:
  name: 'Custom Distribution'
  install:
    theme: gin
    finish_url: '/?tour=1'

Initially the drupalSettings for gin is simply missing, breaking the frontend.

Then as the installation goes, it fails to process other parts as well.

Proposed resolution

Add gin_preprocess_install_page() hook that sets the default drupalSettings for gin.

Add InstallerKernel::installationAttempted() checks around certain preprocess hooks e.g. gin_preprocess_html so that it bails early and doesn't attempt to do any of the extra customization logic.

Remaining tasks

Provide MR

User interface changes

N/A