Fixes #554

Allows you to wrap sources in quotes, meaning dead sources can be added like so:

{
  "post": {
    "source_diff": "\"-test source\""
  }
}

and removed (oddly) like so:

{
  "post": {
    "source_diff": "-\"-test source\""
  }
}

Yeah, it's weird, but it's better than not being able to do it.

This isn't required though, you can add/remove regular sources exactly the same as before. This does however mean no sources can start or end with quotes (when added with source_diff), which at the moment affects 40 posts:
https://e621.net/posts?tags=~source:\"* ~source:*\"

and I'd argue all of them are invalid sources anyways...

Alternatively, the prefix/suffix can be anything, it doesn't need to be quotes.