Check max upload size and server space availability before uploading a new file

[rperezb]

AC:

• Before uploading a new file it has to be checked whether the file is not bigger than the max upload size or the available space on the server
• In case it will be bigger, the upload will be cancelled and the user will be warned with an error

[nasli]

The current error show "Waiting for server connection" (even if the server is connected).
This will replace with an accurate error message.

[davivel]

@michaelstingl , is this still a green ticket?

[michaelstingl]

I don't think so. @butonic You raised priority. Please ping me to let me know the urgency.

[davivel]

Lack of quota in uploads was already handled in #822 , though after the upload instead of before. That should cover the 'urgency' of the topic, so this can go to the release after.