[Technical Debt] use signed urls for all media fetching

[kulmann]

There are some locations in code where we actually don't use signed urls for fetching media as of now.

• Files app: FileItem.vue loads thumbnail images as blob
• Mediaviewer app: Mediaviewer.vue loads all image files as blob.

In all cases, the mixin mediaSource.js is used, so it is fairly easy to change. But with the current implementation of signed urls in oc10 and ocis, all requests will fetch a new signing key first. Because of this, all requests are actually firing two requests, which slows down loading thumbnails in the files list a lot.

Solution would be, to properly cache the signing key. Requires both oc10 and ocis to expose the token TTL, so that the client can cache the signing key. Then we could try again to switch over to loading everything from a signed url instead of as a blob.

[fschade]

some points mentioned where fixed by #5194. We should revisit this issue and discuss further improvements