[media-viewer] Reloading does not work

[dschmidt]

Steps to reproduce

1. Open an image in the media-viewer
2. Reload the page

Expected behaviour

The same image that was shown before the reload is shown again

Actual behaviour

MediaViewer triggers loadFolder action which fails because it was removed in #4627, more specifically in 138a492.
A broken MediaViewer is shown, but at least it can be closed by clicking the x.

loadFolder is still used in
https://github.com/owncloud/web/blob/master/packages/web-app-media-viewer/src/mixins/loader.js#L31
https://github.com/owncloud/web/blob/master/packages/web-runtime/src/components/Notifications.vue#L67

This issue is caused by the former appearance, more issues might be caused by the latter - but I don't know :-)

When we fix it, we should add tests to avoid another regression further down the road...

@LukasHirt (as you replaced the loadFolder with new code:) is this a no-brainer for you to fix?
Then I'd be happy if you could fix it. If not, I'd still be happy for pointers and I can probably take care of it

[LukasHirt]

It should be a no-brainer, I can take care of it