Use urls for uploads in the store and file browser

A part of the refactoring for #828 (closed). The store is updated to use installationUrl + uploadId, rather than just uploadId to manage uploads, and the file browser adaptors are updated so they take absolute upload urls as the main argument (containing information about installationUrl + uploadId + rawPath), rather than just uploadId + rawPath. Some additional work still needs to be done before we can actually fetch data from external installations, though, so only urls pointing to the local installations work for now.