Require Slack app settings when Slack app is enabled

What does this MR do and why?

This change requires that all of the Slack app settings are provided for the GitLab for Slack app to be enabled for an instance.

As the settings only display for GitLab.com?, self-managed should not have the slack_app_enabled setting enabled currently.

gitlab.com, staging.gitlab.com, and staging-ref.gitlab.com instances have the slack_app_enabled setting enabled. These instances have all of these settings present so this change will not make their settings invalid. See Slack messages for staging-ref and staging (internal, good for 90 days), and we know gitlab.com has all settings enabled because the features for the GitLab for Slack app work.

This change is in preparation for making the GitLab for Slack app available for self-managed GitLab instances &1211 (closed) by making all of the settings necessary for Slack app operations required if the slack_app_enabled setting is enabled.

#361595 (closed)

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Related to #361595 (closed)