refactor: Separate StatusCookie and OperationStatus

[angrynode]

Here i try to make error reporting more flexible:

• OperationStatus can contain a stringy message or a typed error
• OperationStatus error sources will be flattened into a list of strings for display (maybe that's not the best approach but that's already what we're doing for AppStateError)
• StatusCookie can be extracted directly in route handlers to display the error with StatusCookie::with_template for any template that implements FallibleTemplate (to set the status in the template)
• when producing the status to pass to another page, the CookieJar should be extracted from the route then use StatusCookie::error or StatusCookie::success, then you can produce the redirect with StatusCookie::redirect
• added type aliases FlashRedirect and FlashTemplate<Template>

Not sure if that's the best approach in the long run (eg. for i18n) but for now that should make it easier:

• to add new pages producing/consuming cookie-based flash messages
• to add new pages with an OperationStatus that's not based on a cookie (useful for feat: Start supporting magnet upload #17)

[angrynode]

Is that a proper display for nested errors?