Make development seed for application settings idempotent

What does this MR do and why?

This enables users to run FILTER=application_settings bin/rake db:seed_fu as many times as they want, and allows retries to happen in CI if needed.

This also reduces the number of updates to the application settings. Each update! causes a validation to occur. Each URL will be validated by resolving its DNS address, which could be slow or time out.

Relates to gitlab-development-kit#2062 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.