Convert archive_rate_limit feature flag to Application Setting

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Context

There are two issues to solve:

1. archive_rate_limit is meant to be a temporary feature flag. It's enabled in Gitlab.com.
2. There is no way to configure the project_repositories_archive rate - it's currently hard-coded to 5 per 1.minute.

Requirements

Remove the archive_rate_limit feature flag in favor of an application setting for project_repositories_archive.

Implementation plan

Follow this guideline to add the new application setting. There are a list of steps that you should follow.