Don't allow duplicate resource links (by URL) in incidents

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In incidents, you can add duplicate resource links:

This doesn't seem very useful to allow, and could be confusing when the link text is different.

Implementation guide

1. Return an error saying "URL already linked" in IncidentManagement::IssuableResourceLinks::Create
   • For this, we need to check if the incident already links a resource with the same link
     • For the sake of simplicity we are not normalizing URLs which means that these URLs are identical: https://example.org and https://example.org/ (note the trailing slash)
   • For example, incident.issuable_resource_links.where(link: params[:link]).exists?
   • Check if the existing index (index_issuable_resource_links_on_issue_id) is speedy
   • 🅰️ If not (due to too many linked resources per issue)
     • Consider a composite index on issue_id, link
     • Limit amount of resources per issue to 100. This would also have some positive implications on the UX. See splattael/mwps#21.
2. 🅱️ Ensure that the service response's message is properly propagated to the user. Currently, we fail to say why saving a resource link is failing.

• The locations are:
  • resource_link = ::IncidentManagement::IssuableResourceLinks::CreateService.new(quick_action_target,
  • IssuableResourceLinks::CreateService.new(@incident,
  • response ::IncidentManagement::IssuableResourceLinks::CreateService.new(incident, current_user, args).execute

Note that 🅰️ and 🅱️ could be separate MRs.

Additional guidance:

• Here is how to add a linked resource to an incident
• Note that Linked resources are available in GitLab Premium. Here's a handbook page on how you can get one