It's hard to configure multiple POST URLs per asset type

[rschamp]

When remixing or creating a project as a copy, we use query strings to modify the URL, e.g. /?is_copy=1&title=My Project Title. Since we only rely on storage looking up the right handler based on the asset id, we don't have a way to dynamically set parameters for a particular store().

We need to do some thinking on the design of store since it now doesn't work ideally for either of the servers we're using it for. Our config for assets hack around the current design, which was created around project creation: https://github.com/LLK/scratch-gui/blob/develop/src/lib/storage.js#L24-L28, and we aren't using it at all for project creation anymore because of this URL issue.