ExternalMediaLink: If the URL is already used in another ExternalMediaLink, display an error rather than create a duplicate

[x42bn6]

Sometimes, a user is not always aware of when a URL is added as an ExternalMediaLink by another user, so they end up creating a duplicate.

Examples:

• https://discord.com/channels/93055209017729024/765984788107624468/1474810169346228234
• https://discord.com/channels/93055209017729024/765984788107624468/1418001760974536765

I think the easiest solution would be for the module to check if an existing ExternalMediaLink exists for that URL, and if so, error out.

[hjpalpha]

fyi:

• the form itself doesn't use a module and i think we can not add such a check to the form itself
• we could only let the EMLWrapper error on the created Data Page

[hjpalpha]