Use instance defaults for source branch removal

What does this MR do and why?

Contributes to #465720 (closed)

Problem

The source branch removal option is hard-coded to enabled for new projects, so administrators cannot choose a different instance default.

Solution

• Store the source branch removal default in application settings.
• Use the instance default when creating projects and forks.
• Expose the setting through the application settings API.
• Document and test the setting across models, services, and the API.

References

Relates to #465720 (closed)

Stack order:

1. !260679 (merged): database foundation
2. This MR: backend setting, project default, and API
3. 465720-default-remove-source-branch-admin-ui

How to set up and validate locally

1. In a Rails console, set the instance default:

   ApplicationSetting.current.update!(
     default_remove_source_branch_after_merge: false
   )

2. Create a new project and verify that source branch removal is disabled by default.
3. Create a fork and verify that it uses the instance default.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist.