Enforce Storage Quota already before upload

[fm3]

We should determine the size of the to-uploaded files already in the frontend and check if they exceed the quota.

reserveUpload should send either a total file size or even a file-size per file map. the reserve call should be denied if the sum is too large.

also, the actual uploadChunk requests should be denied if they try to write more than reserved.

[normanrz]

We should also enforce that an upload does not contain more bytes than were reserved.