Move ApplicationSetting-related code out of CurrentSetting class

What does this MR do and why?

This is a follow-up on !138970 (merged).

Related issue: #417763 (closed)

Currently, we have settings in ApplicationSetting class. These are accessed using Gitlab::CurrentSetting class. With cells, we will have settings that will move from ApplicationSetting to OrganizationSetting. But we want to keep accessing settings using Gitlab::CurrentSetting. So we want to move some specific ApplicationSetting related logic to ApplicationSettingFetcher which was introduced in the previous MR

This MR will move a few methods to ApplicationSettingFetcher. The result is that this class has no dependencies on ApplicationSetting model anymore and can now be extended to support OrganizationSetting, which will be a next MR.

Methods that are now moved to ApplicationSettingFetcher:

• (private method) uncached_application_settings
• expire_current_application_settings
• current_application_settings?

Method code was not changed, only copied.

How to set up and validate locally

Test: Modifying ApplicationSetting

• http://localhost:3000 login as administrator
• Ensure the warning about "Check your sign-up restrictions" is displayed and click 'Deactivate'
• http://localhost:3000/admin/application_settings/general#js-signup-settings Remove the checkbox "Sign-up enabled" and click "Save Changes" button
• The warning should now be removed.
• http://localhost:3000/admin/application_settings/general#js-signup-settings Set the checkbox "Sign-up enabled" and click "Save Changes" button
• The warning should be back.

Screen_Recording_2024-01-03_at_17.09.00

Test: Ensure rake tasks can run even without ApplicationSetting available

A suitable Rake task is rake db:setup which will create databases

Run this sequence (NOTE This will wipe out the local development databases)

# Drop all test/development databases (please backup them if you have valuable data)
gdk psql -d postgres -c "drop database gitlabhq_development;"
gdk psql -d postgres -c "drop database gitlabhq_development_ci;"
gdk psql -d postgres -c "drop database gitlabhq_test;"
gdk psql -d postgres -c "drop database gitlabhq_test_ci;"

# Run this rake task: it should run, even without ApplicationSetting in the database
bundle exec rake db:setup

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.