Add allow_account_deletion to ApplicationSettings

What does this MR do and why?

Related to issue #26053 (closed)

Add a configuration to ApplicationSettings to control whether users can delete their account. The feature is only available in EE version.

This is in the Admin -> General -> Account and limit section.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. Sign up a new user A and sign in.
2. Go to the Preferences -> Account, click Delete account.
3. You can delete the account A. (That's what we expect).
4. Sign up a new user B and sign in.
5. Open another browser window and sign in with admin account.
6. Go to Admin -> General -> Account and limit section.
7. Uncheck the checkbox Allows users to delete their own accounts and click Save changes.
8. Enable feature flag deleting_account_disabled_for_users.
9. Go back to the browser window with user B.
10. Go to the Preferences -> Account, click Delete account.
11. You cannot find the Delete account section.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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