Need to support duping between websites

[aluhrs13]

• [x] Handle multiple SourceSites in Details and Edit pages
• [ ] Handle multiple SourceSites in view API and beta site
• [ ] Ensure every Mini has a MiniSourceSite
• [ ] Check for existing name & creator match on mini submission
• [ ] Figure out how to mass de-dupe already existing items.

[anaximander23]

The neatest way to do this is probably for a Mini to have a collection of sources, rather than just a single link. This allows us to represent a Mini as a thing that is found on multiple sites, and lets us use the metadata from all of those sites to help suggest tags for it - which is useful if some sites have better systems for categorising, tagging, etc. as we can piggyback off that information.

[anaximander23]

My current branch feature/source-site-polymorphism can lay some groundwork for this.