[TASK] Add information on uploading extension version without composer.json

A new query argument composerStatus is now being added on redirecting to the
extension overview (RegisterKey->indexAction).

In case the argument contains either missing (no composer.json in uploaded version) or
missingExtensionKey (no extension key in composer.json of uploaded version), a warning
is being displayed, informing the extension owner about it.

Note: This only affects the GUI and neither the SOAP nor the REST API.

Closes #516 (closed)