This merges the farm_settings module into farm_setup, uninstalls farm_settings via an update hook, and marks it as obsolete.

A companion branch against farmOS 4.x does the same thing, but also removes the farm_settings module and the update hook (adding its name to hook_removed_post_updates() to ensure hosts upgrade to the latest 3.x release before 4.x - see #962): 4.x...mstenta:farmOS:4.x-merge-settings-into-setup

The motivation for doing this is twofold:

1. Right now the modules form is in farm_settings, but we are considering adding a "Setup Wizard", which would be in farm_setup, so it feels logical for them to be in the same module. We can't make farm_setup depend on farm_settings because that would create a circular dependency.
2. It feels more elegant to have one module. The distinction between the two is hard to describe, and sometimes hard to remember 😆. And the "settings form" feel like a subset of the "setup process/options".

This was briefly considered when the farm_setup module was first added in #706. See: #706 (comment)

Right now they are both pretty simple. farm_setup basically just provides the "Setup" main menu item, and farm_settings depends on farm_setup (for that menu item) and adds two forms: the settings form+menu item and the module installation form. So as it is, it's possible to have farm_setup installed without farm_settings, but it isn't possible to have farm_settings without farm_setup. They have separate permissions (access farm setup vs administer farm settings) - so it's already possible to set up users with the ability to access Setup without accessing Setup > Settings.

With all of that in mind, I don't see a strong reason to keep them separate, and merging them simplifies next steps for the setup wizard work.

PS: There is a second commit which simply moves the Gin-specific logic of adding the form-checkboxes class to the modules form over to the farm_ui_theme module, which is responsible for encapsulating all Gin-specific stuff.